loop: don't destroy lo->workqueue in __loop_clr_fd

There is no need to destroy the workqueue when clearing unbinding
a loop device from a backing file.  Not doing so on the other hand
avoid creating a complex lock dependency chain involving the global
system_transition_mutex.

Based on a patch from Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>.

Reported-by:  <syzbot+6479585dfd4dedd3f7e1@syzkaller.appspotmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by:  <syzbot+6479585dfd4dedd3f7e1@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/20220330052917.2566582-16-hch@lst.de


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