veth: move loopback logic to common location

The veth driver contains code to forward an skb
from the start_xmit function of one network
device into the receive path of another device.

Moving that code into a common location lets us
reuse the code for direct forwarding of data
between macvlan ports, and possibly in other
drivers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 97873e3..9428793 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1562,6 +1562,8 @@
 extern int		dev_hard_start_xmit(struct sk_buff *skb,
 					    struct net_device *dev,
 					    struct netdev_queue *txq);
+extern int		dev_forward_skb(struct net_device *dev,
+					struct sk_buff *skb);
 
 extern int		netdev_budget;