loop: initialize the worker tracking fields once

There is no need to reinitialize idle_worker_list, worker_tree and timer
every time a loop device is configured.  Just initialize them once at
allocation time.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Tested-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20220330052917.2566582-8-hch@lst.de


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