loop: fix LOOP_GET_STATUS lock imbalance

Commit 2d1d4c1e

 made loop_get_status() drop lo_ctx_mutex before
returning, but the loop_get_status_old(), loop_get_status64(), and
loop_get_status_compat() wrappers don't call loop_get_status() if the
passed argument is NULL. The callers expect that the lock is dropped, so
make sure we drop it in that case, too.

Reported-by:  <syzbot+31e8daa8b3fc129e75f2@syzkaller.appspotmail.com>
Fixes: 2d1d4c1e

 ("loop: don't call into filesystem while holding lo_ctl_mutex")
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>