virtio-balloon: correct the comment of virtballoon_migratepage()

After commit 68f2736a ("mm: Convert all PageMovable users to
movable_operations"), the execution path has been changed to

move_to_new_folio
	movable_operations->migrate_page
		balloon_page_migrate
			balloon_page_migrate->balloon_page_migrate
				balloon_page_migrate

Correct the outdated comment.

Signed-off-by: Xueshi Hu <xueshi.hu@smartx.com>
Message-Id: <20230813140709.835536-1-xueshi.hu@smartx.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>