loop: de-duplicate the idle worker freeing code

Use a common helper for both timer based and uncoditional freeing of idle
workers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20220330052917.2566582-7-hch@lst.de


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