generic: test fsync against a removed directory

generic: test fsync against a removed directory

Test that if we fsync a directory that became empty after removing it
and we have a power failure, we are able to mount the filesystem and the
directory does not exists anymore.

This exercises a bug found in btrfs which is fixed by the following patch:

  btrfs: fix missing last_unlink_trans update when removing a directory

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Zorro Lang <zlang@kernel.org>