loop: fix LOOP_GET_STATUS lock imbalance

loop: fix LOOP_GET_STATUS lock imbalance

Commit 2d1d4c1e591f 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: [email protected]
Fixes: 2d1d4c1e591f ("loop: don't call into filesystem while holding lo_ctl_mutex")
Signed-off-by: Omar Sandoval <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>