loop: split loop_control_ioctl

Split loop_control_ioctl into a helper for each command.  This keeps the
code nicely separated for the upcoming locking changes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Link: https://lore.kernel.org/r/20210623145908.92973-6-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>