macvlan: Support creating macvtaps from macvlans

When running in a network namespace whose only link to the outside
world is a macvlan device, not being able to create a macvtap off of
it is a real pain.

So modify macvtap creation to automatically forward a creation of a
macvtap on a macvlan to become a creation of a macvtap on the
underlying network device, just like is currently done with
macvlan-on-macvlan devices.

v2: Use netif_is_macvlan and macvlan_dev_real_dev helpers to make it
    more clear what we're doing.
Signed-off-by: Kevin Wallace <kevin@pentabarf.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macvlan.c

@@ -820,13 +820,11 @@ int macvlan_common_newlink(struct net *src_net, struct net_device *dev,
 	if (lowerdev == NULL)
 		return -ENODEV;
 
-	/* When creating macvlans on top of other macvlans - use
+	/* When creating macvlans or macvtaps on top of other macvlans - use
 	 * the real device as the lowerdev.
 	 */
-	if (lowerdev->rtnl_link_ops == dev->rtnl_link_ops) {
-		struct macvlan_dev *lowervlan = netdev_priv(lowerdev);
-		lowerdev = lowervlan->lowerdev;
-	}
+	if (netif_is_macvlan(lowerdev))
+		lowerdev = macvlan_dev_real_dev(lowerdev);
 
 	if (!tb[IFLA_MTU])
 		dev->mtu = lowerdev->mtu;