btrfs: defrag: also check PagePrivate for subpage cases in cluster_pages_for_defrag()

In function cluster_pages_for_defrag() we have a window where we unlock
page, either start the ordered range or read the content from disk.

When we re-lock the page, we need to make sure it still has the correct
page->private for subpage.

Thus add the extra PagePrivate check here to handle subpage cases
properly.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -1278,7 +1278,8 @@ static int cluster_pages_for_defrag(struct inode *inode,
 			 * we unlocked the page above, so we need check if
 			 * it was released or not.
 			 */
-			if (page->mapping != inode->i_mapping) {
+			if (page->mapping != inode->i_mapping ||
+			    !PagePrivate(page)) {
 				unlock_page(page);
 				put_page(page);
 				goto again;
@@ -1296,7 +1297,7 @@ static int cluster_pages_for_defrag(struct inode *inode,
 			}
 		}
 
-		if (page->mapping != inode->i_mapping) {
+		if (page->mapping != inode->i_mapping || !PagePrivate(page)) {
 			unlock_page(page);
 			put_page(page);
 			goto again;