xen-netfront: re-order error checks in xennet_get_responses()

Check the retrieved grant reference first; there's no point trying to
have xennet_move_rx_slot() move invalid data (and further defer
recognition of the issue, likely making diagnosis yet more difficult).
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/xen-netfront.c

@@ -1043,16 +1043,6 @@ static int xennet_get_responses(struct netfront_queue *queue,
 	}
 
 	for (;;) {
-		if (unlikely(rx->status < 0 ||
-			     rx->offset + rx->status > XEN_PAGE_SIZE)) {
-			if (net_ratelimit())
-				dev_warn(dev, "rx->offset: %u, size: %d\n",
-					 rx->offset, rx->status);
-			xennet_move_rx_slot(queue, skb, ref);
-			err = -EINVAL;
-			goto next;
-		}
-
 		/*
 		 * This definitely indicates a bug, either in this driver or in
 		 * the backend driver. In future this should flag the bad
@@ -1066,6 +1056,16 @@ static int xennet_get_responses(struct netfront_queue *queue,
 			goto next;
 		}
 
+		if (unlikely(rx->status < 0 ||
+			     rx->offset + rx->status > XEN_PAGE_SIZE)) {
+			if (net_ratelimit())
+				dev_warn(dev, "rx->offset: %u, size: %d\n",
+					 rx->offset, rx->status);
+			xennet_move_rx_slot(queue, skb, ref);
+			err = -EINVAL;
+			goto next;
+		}
+
 		if (!gnttab_end_foreign_access_ref(ref)) {
 			dev_alert(dev,
 				  "Grant still in use by backend domain\n");