virtio_net: Set the mac config only when VIRITO_NET_F_MAC

VIRTIO_NET_F_MAC indicates the presence of the mac field in config
space, not the validity of the value it contains.  Allow the mac to be
changed at runtime, but only push the change into config space with the
VIRTIO_NET_F_MAC feature present.
Signed-off-by: Alex Williamson <alex.williamson@hp.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -575,8 +575,9 @@ static int virtnet_set_mac_address(struct net_device *dev, void *p)
 	if (ret)
 		return ret;
 
-	vdev->config->set(vdev, offsetof(struct virtio_net_config, mac),
-			  dev->dev_addr, dev->addr_len);
+	if (virtio_has_feature(vdev, VIRTIO_NET_F_MAC))
+		vdev->config->set(vdev, offsetof(struct virtio_net_config, mac),
+		                  dev->dev_addr, dev->addr_len);
 
 	return 0;
 }
@@ -876,11 +877,8 @@ static int virtnet_probe(struct virtio_device *vdev)
 		vdev->config->get(vdev,
 				  offsetof(struct virtio_net_config, mac),
 				  dev->dev_addr, dev->addr_len);
-	} else {
+	} else
 		random_ether_addr(dev->dev_addr);
-		vdev->config->set(vdev, offsetof(struct virtio_net_config, mac),
-				  dev->dev_addr, dev->addr_len);
-	}
 
 	/* Set up our device-specific information */
 	vi = netdev_priv(dev);