Description of problem:
The mount path of subpath does not get the setgid bit by fsGroup.

Version-Release number of selected component (if applicable):
openshift v3.9.9
kubernetes v1.9.1+a0ce1bc657
etcd 3.2.16

How reproducible:
Very

Steps to Reproduce:
1. Create a Pod with emptyDir volume
kind: Pod
apiVersion: v1
metadata: { name: "subpath" }
spec:
  volumes:
    - { name: "direct",  emptyDir: {}}
    - { name: "subpath", emptyDir: {}}
  securityContext:
    fsGroup: 100
    # games(35) is member in group users(100)
    runAsUser: 35
  containers:
    - image: aosqe/hello-openshift
      name: show
      volumeMounts:
        - name: "direct"
          mountPath: "/mnt/direct"
        - name: "subpath"
          mountPath: "/mnt/subpath"
          subPath: "a"
2. oc rsh kind: Pod
apiVersion: v1
metadata: { name: "subpath" }
spec:
  volumes:
    - { name: "direct",  emptyDir: {}}
    - { name: "subpath", emptyDir: {}}
  securityContext:
    fsGroup: 100
    # games(35) is member in group users(100)
    runAsUser: 35
  containers:
    - image: aosqe/hello-openshift
      name: show
      volumeMounts:
        - name: "direct"
          mountPath: "/mnt/direct"
        - name: "subpath"
          mountPath: "/mnt/subpath"
          subPath: "a"
2. oc exec subpath -- ls /mnt -l 

Actual results:
# oc exec subpath -- ls /mnt -l      
total 0
drwxrwsrwx    2 root     users            6 Mar 15 08:09 direct
drwxrwxrwx    2 root     users            6 Mar 15 08:09 subpath

Noticed the setgid bit is x instead of s.

Expected results:
The subpath mount path should have setgid bit.