loop: reduce loop_ctl_mutex coverage in loop_exit

loop_ctl_mutex is only needed to iterate the IDR for removing the loop
devices, so reduce the coverage.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Link: https://lore.kernel.org/r/20210623145908.92973-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>