Bug 2245261

Description of problem:

Epic: https://issues.redhat.com/browse/RHSTOR-4851
`nfs: add support for shallow volumes - Dev Preview`
The above epic aims to add support for shallow volumes -> 
snaphots mounted as readonly volumes for cephfs subvolumes exported over NFS.

Design for CephFS shallow volume: https://github.com/ceph/ceph-csi/blob/devel/docs/design/proposals/cephfs-snapshot-shallow-ro-vol.md#snapshots-as-shallow-read-only-volumes

We need to be able to mount nfs exports of cephfs subvolumes with the path `/volumes/<VOLUME NAME>/<SUBVOLUME NAME>/.snap/...` to access snapshot contents.

However, `/.snap` does not show up in a mounted subvolume nfs export.
(it can be accessed when root '/' is exported and mounted).

https://docs.ceph.com/en/quincy/mgr/nfs/#create-cephfs-export

The main goal is to facilitate backup softwares by replacing expensive full clones with shallow volumes(snapshot as readonly volume).

How reproducible:
Everytime

Steps to Reproduce:

1. Export a snapshot with path  `/volumes/<VOLUME NAME>/<SUBVOLUME NAME>/.snap/...`
[root@m1 /]# ceph nfs export create cephfs my-nfs /snap myfs /volumes/csi/csi-vol-5a989f69-a58a-4a7f-b803-a91c596559d8/.snap/csi-snap-ca8efd01-a71f-4c6d-978f-4a8cc28 --readonly
bbedc/fa15a4bc-2177-4c14-b449-253152c1c20e
{
    "bind": "/snap",
    "fs": "myfs",
    "path": "/volumes/csi/csi-vol-5a989f69-a58a-4a7f-b803-a91c596559d8/.snap/csi-snap-ca8efd01-a71f-4c6d-978f-4a8cc28bbedc/fa15a4bc-2177-4c14-b449-253152c1c20e",
    "cluster": "my-nfs",
    "mode": "RO"
}

2. try to mount the export
[root@m1 /]# mkdir /snap
[root@m1 /]# mount -t nfs4 -o snap=tcp rook-ceph-nfs-my-nfs-a:/snap /snap
mount.nfs4: mounting rook-ceph-nfs-my-nfs-a:/snap failed, reason given by server: No such file or directory

Actual results:
The mount fails.

Expected results:
The mount should success and data within the snapshot should be accessible.

Additional info:

- https://issues.redhat.com/browse/RHSTOR-4851
- https://github.com/ceph/ceph-csi/blob/devel/docs/design/proposals/cephfs-snapshot-shallow-ro-vol.md#snapshots-as-shallow-read-only-volumes
- https://github.com/ceph/ceph-csi/issues/3997