loop: make autoclear operation asynchronous

loop: make autoclear operation asynchronous

syzbot is reporting circular locking problem at __loop_clr_fd() [1], for
commit 87579e9b7d8dc36e ("loop: use worker per cgroup instead of kworker")
is calling destroy_workqueue() with disk->open_mutex held.

This circular dependency cannot be broken unless we call __loop_clr_fd()
without holding disk->open_mutex. Therefore, defer __loop_clr_fd() from
lo_release() to a WQ context.

Link: https://syzkaller.appspot.com/bug?extid=643e4ce4b6ad1347d372
Reported-by: syzbot <[email protected]>
Suggested-by: Christoph Hellwig <[email protected]>
Cc: Jan Kara <[email protected]>
Tested-by: [email protected]
Signed-off-by: Tetsuo Handa <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>