xen-netback: correctly check failed allocation

Since vzalloc can be failed in memory pressure,
writes -ENOMEM to xenstore to indicate error.
Signed-off-by: NInsu Yun <wuninsu@gmail.com>
Acked-by: NWei Liu <wei.liu2@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/xenbus.c

@@ -788,6 +788,12 @@ static void connect(struct backend_info *be)
 	/* Use the number of queues requested by the frontend */
 	be->vif->queues = vzalloc(requested_num_queues *
 				  sizeof(struct xenvif_queue));
+	if (!be->vif->queues) {
+		xenbus_dev_fatal(dev, -ENOMEM,
+				 "allocating queues");
+		return;
+	}
+
 	be->vif->num_queues = requested_num_queues;
 	be->vif->stalled_queues = requested_num_queues;