fstests: Verify dir permissions when creating a stub subvolume

fstests: Verify dir permissions when creating a stub subvolume

btrfs supports creating nesting subvolumes however snapshots are not
recurive. When a snapshot is taken of a volume which contains a subvolume
the subvolume is replaced with a stub subvolume which has the same name and
uses inode number 2. This test validates that the stub volume copies
permissions of the original volume.

Signed-off-by: Lee Trager <lee@trager.us>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>