xen-netfront, xen-netback: Use correct minimum MTU values

RFC791 specifies the minimum MTU to be 68, while xen-net{front|back}
drivers use a minimum value of 0.

When set MTU to 0~67 with xen_net{front|back} driver, the network
will become unreachable immediately, the guest can no longer be pinged.

xen_net{front|back} should not allow the user to set this value which causes
network problems.

Reported-by: Chen Shi <cheshi@redhat.com>
Signed-off-by: Mohammed Gamal <mgamal@redhat.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/net/xen-netback/interface.c

@@ -486,7 +486,7 @@ struct xenvif *xenvif_alloc(struct device *parent, domid_t domid,
 
 	dev->tx_queue_len = XENVIF_QUEUE_LENGTH;
 
-	dev->min_mtu = 0;
+	dev->min_mtu = ETH_MIN_MTU;
 	dev->max_mtu = ETH_MAX_MTU - VLAN_ETH_HLEN;
 
 	/*

drivers/net/xen-netfront.c

@@ -1316,7 +1316,7 @@ static struct net_device *xennet_create_dev(struct xenbus_device *dev)
 	netdev->features |= netdev->hw_features;
 
 	netdev->ethtool_ops = &xennet_ethtool_ops;
-	netdev->min_mtu = 0;
+	netdev->min_mtu = ETH_MIN_MTU;
 	netdev->max_mtu = XEN_NETIF_MAX_TX_SIZE;
 	SET_NETDEV_DEV(netdev, &dev->dev);