loop: Switch to set_capacity_revalidate_and_notify()

This was recently added to block/genhd.c, and takes care of both
updating the capacity and notifying userspace of the new size.

Signed-off-by: Martijn Coenen <maco@android.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>