Merge pull request #15294 from adelton/flex-bind-mount

Automatic merge from submit-queue

UPSTREAM: 49118: Allow unmounting bind-mounted directories.

For files, we cannot use `path/..`;
we could use `filepath.Dir` but for bind-mounted, `isNotMounted`
which calls `IsLikelyNotMountPoint` would not work anyway.
Let's just have the driver do the work.

Addressing
```
Error: UnmountVolume.TearDown failed for volume "..." (volume.spec.Name: "...") pod "..." (UID: "...") with: lstat /path/.../test-flex/..: not a directory
```