virtio-balloon: correct the comment of virtballoon_migratepage()

After commit 68f2736 ("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Correct the outdated comment.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 <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Xuan Zhuo <[email protected]>

Author: Martins3

drivers/virtio/virtio_balloon.c

@@ -745,7 +745,7 @@ static void report_free_page_func(struct work_struct *work)
  *  2) update the host about the old page removed from vb->pages list;
  *
  * This function preforms the balloon page migration task.
- * Called through balloon_mapping->a_ops->migratepage
+ * Called through movable_operations->migrate_page
  */
 static int virtballoon_migratepage(struct balloon_dev_info *vb_dev_info,
 		struct page *newpage, struct page *page, enum migrate_mode mode)