loop: move loop_ctl_mutex locking into loop_add

Move acquiring and releasing loop_ctl_mutex from the callers into
loop_add.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Link: https://lore.kernel.org/r/20210623145908.92973-7-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>