vxlan: Update hard_header_len based on lowerdev when instantiating VXLAN

In the event of a VXLAN device being linked to a device that has a
hard_header_len greater than that of standard ethernet we could end up with
the hard_header_len not being large enough for outgoing frames.  In order to
prevent this we should update the length when a lowerdev is provided.
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -1102,6 +1102,10 @@ static int vxlan_newlink(struct net *net, struct net_device *dev,
 
 		if (!tb[IFLA_MTU])
 			dev->mtu = lowerdev->mtu - VXLAN_HEADROOM;
+
+		/* update header length based on lower device */
+		dev->hard_header_len = lowerdev->hard_header_len +
+				       VXLAN_HEADROOM;
 	}
 
 	if (data[IFLA_VXLAN_TOS])