netback: use correct index for invalidation in netbk_tx_check_mop()

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/xen/netback/netback.c
+++ b/drivers/xen/netback/netback.c
@@ -1155,7 +1155,7 @@ static int netbk_tx_check_mop(struct sk_
 		pending_idx = *((u16 *)skb->data);
 		netif_idx_release(pending_idx);
 		for (j = start; j < i; j++) {
-			pending_idx = (unsigned long)shinfo->frags[i].page;
+			pending_idx = (unsigned long)shinfo->frags[j].page;
 			netif_idx_release(pending_idx);
 		}