loop: let set_capacity_revalidate_and_notify update the bdev size

There is no good reason to call revalidate_disk_size separately.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>