Description of problem:
When we create a migration plan to execute an intra-cluster migration from OCS4 (rbd) storage class to OCS4(rbd) storage class the migration plan is reporting this warning "Failed gathering extended PV usage information for PVs [test-data-app1], please see MigAnalytic openshift-migration/local-tvq7x for details"

Version-Release number of selected component (if applicable):
TARGET CLUSTER: AWS OCP 4.9 (MTC 1.6.0) OCS4  (controller)
SOURCE CLUSTER: AWS OCP 3.11 (MTC 1.5.1) NFS
RREPLICATION REPOSITORY: AWS S3

How reproducible:
Always

Steps to Reproduce:
1. Create an applicatin with a pvc in the OCP 4.9 cluster. Use rbd OCS4 storage class

the pvc is like this:
$ oc get pvc
NAME             STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS                  AGE
test-data-app1   Bound    pvc-413d08ca-382d-4a44-97d4-0d3ddc95def9   1Gi        RWO            ocs-storagecluster-ceph-rbd   43m


2. Create a migration plan to execute an inter-cluster migration. From OCP4.9 to the same OCP4.9 cluster. Use rbd OCS4 as the destination storage class too.


Actual results:
The migration plan is reporting a miganalytic warning
  - category: Warn
    lastTransitionTime: "2021-09-08T14:43:12Z"
    message: Failed gathering extended PV usage information for PVs [test-data-app1],
      please see MigAnalytic openshift-migration/local-tvq7x for details
    reason: FailedRunningDf
    status: "True"
    type: ExtendedPVAnalysisFailed

Expected results:
Miganalytic should have no problem gathering the PV usage information

Additional info:

This is the miganalytic executed command
{"level":"info","ts":1631112159.3300958,"logger":"analytics","msg":"Executing df command inside source cluster Restic Pod to measure actual usage for extended PV analysis","migAnalytic":"local-tvq7x","pod":"openshift-migration/restic-8tns9","command":["/bin/bash","-c","df -BMB /host_pods/cb95d243-5187-43fb-8882-0d102b333727/volumes/*/pvc-413d08ca-382d-4a44-97d4-0d3ddc95def9"]}


This is the mounted volume in the pod:

$ df -h
Filesystem                Size      Used Available Use% Mounted on
/dev/rbd0               975.9M    602.5M    357.4M  63% /data/test 


This is the full migration plan:
apiVersion: migration.openshift.io/v1alpha1
kind: MigPlan
metadata:
  annotations:
    openshift.io/touch: 17bea80f-10b3-11ec-afae-0a580a800226
  creationTimestamp: "2021-09-08T14:42:36Z"
  generation: 5

  name: local
  namespace: openshift-migration
  resourceVersion: "232299"
  uid: c7a2ea14-af25-4e24-aac3-4e88160a8a4e
spec:
  destMigClusterRef:
    name: host
    namespace: openshift-migration
  indirectImageMigration: true
  indirectVolumeMigration: true
  migStorageRef:
    name: automatic
    namespace: openshift-migration
  namespaces:
  - ocp-44561-state-intralocal
  persistentVolumes:
  - capacity: 1Gi
    name: pvc-413d08ca-382d-4a44-97d4-0d3ddc95def9
    proposedCapacity: "0"
    pvc:
      accessModes:
      - ReadWriteOnce
      hasReference: true
      name: test-data-app1:test-data-app1
      namespace: ocp-44561-state-intralocal
    selection:
      action: copy
      copyMethod: filesystem
      storageClass: ocs-storagecluster-ceph-rbd
    storageClass: ocs-storagecluster-ceph-rbd
    supported:
      actions:
      - skip
      - copy
      copyMethods:
      - filesystem
      - snapshot
  srcMigClusterRef:
    name: host
    namespace: openshift-migration
status:
  conditions:
  - category: Warn
    lastTransitionTime: "2021-09-08T14:42:47Z"
    message: Source namespaces [ocp-44561-state-intralocal] are mapped to namespaces
      which result in conflicts. Stage/Final migrations will be blocked on this migration
      plan. Only state migrations are possible provided there are no conflicts in
      the PVC names.
    reason: ConflictingNamespaces
    status: "True"
    type: IntraClusterMigration
  - category: Required
    lastTransitionTime: "2021-09-08T14:42:47Z"
    message: The `persistentVolumes` list has been updated with discovered PVs.
    reason: Done
    status: "True"
    type: PvsDiscovered
  - category: Required
    lastTransitionTime: "2021-09-08T14:42:47Z"
    message: The storage resources have been created.
    reason: Done
    status: "True"
    type: StorageEnsured
  - category: Warn
    lastTransitionTime: "2021-09-08T14:43:12Z"
    message: Failed gathering extended PV usage information for PVs [test-data-app1],
      please see MigAnalytic openshift-migration/local-tvq7x for details
    reason: FailedRunningDf
    status: "True"
    type: ExtendedPVAnalysisFailed
  - category: Required
    lastTransitionTime: "2021-09-08T14:42:47Z"
    message: The migration plan is ready.
    status: "True"
    type: Ready
  - category: Warn
    lastTransitionTime: "2021-09-08T14:42:47Z"
    message: Source PVCs [ocp-44561-state-intralocal/test-data-app1] are mapped to
      destination PVCs which result in conflicts. Please map each source PVC to a
      distinct destination PVC before running a State Migration.
    reason: NotDistinct
    status: "True"
    type: PvNameConflict
  destStorageClasses:
  - accessModes:
    - ReadWriteOnce
    name: gp2
    provisioner: kubernetes.io/aws-ebs
  - accessModes:
    - ReadWriteOnce
    name: gp2-csi
    provisioner: ebs.csi.aws.com
  - accessModes:
    - ReadWriteOnce
    default: true
    name: ocs-storagecluster-ceph-rbd
    provisioner: openshift-storage.rbd.csi.ceph.com
  - accessModes:
    - ReadWriteOnce
    - ReadOnlyMany
    - ReadWriteMany
    name: ocs-storagecluster-cephfs
    provisioner: openshift-storage.cephfs.csi.ceph.com
  - accessModes:
    - ReadWriteOnce
    name: openshift-storage.noobaa.io
    provisioner: openshift-storage.noobaa.io/obc
  excludedResources:
  - imagetags
  - templateinstances
  - clusterserviceversions
  - packagemanifests
  - subscriptions
  - servicebrokers
  - servicebindings
  - serviceclasses
  - serviceinstances
  - serviceplans
  - operatorgroups
  - events
  - events.events.k8s.io
  observedDigest: 27ca5d44ecab1de03a8f0223595b4cf3044df1bc019434b743f8b335d00557c0
  srcStorageClasses:
  - accessModes:
    - ReadWriteOnce
    name: gp2
    provisioner: kubernetes.io/aws-ebs
  - accessModes:
    - ReadWriteOnce
    name: gp2-csi
    provisioner: ebs.csi.aws.com
  - accessModes:
    - ReadWriteOnce
    default: true
    name: ocs-storagecluster-ceph-rbd
    provisioner: openshift-storage.rbd.csi.ceph.com
  - accessModes:
    - ReadWriteOnce
    - ReadOnlyMany
    - ReadWriteMany
    name: ocs-storagecluster-cephfs
    provisioner: openshift-storage.cephfs.csi.ceph.com
  - accessModes:
    - ReadWriteOnce
    name: openshift-storage.noobaa.io
    provisioner: openshift-storage.noobaa.io/obc