btrfs: regression test for subvol deletion after rename

btrfs: regression test for subvol deletion after rename

Test removal of a subvolume via rmdir after it has been renamed into a
snapshot of the volume that originally contained the subvolume
reference.

This currently fails on btrfs but is fixed by the patch with the title

  "btrfs: fix invalid removal of root ref"

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>