loop: make autoclear operation asynchronous

syzbot is reporting circular locking problem at __loop_clr_fd() [1], for
commit 87579e9b ("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

 [1]
Reported-by: syzbot <syzbot+643e4ce4b6ad1347d372@syzkaller.appspotmail.com>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Tested-by:  <syzbot+643e4ce4b6ad1347d372@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/1ed7df28-ebd6-71fb-70e5-1c2972e05ddb@i-love.sakura.ne.jp


Signed-off-by: Jens Axboe <axboe@kernel.dk>