loop: Use invalidate_disk() helper to invalidate gendisk

Use invalidate_disk() helper to simplify the code for gendisk
invalidation.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210922123711.187-3-xieyongji@bytedance.com


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