net/core: Allow tagged VLAN packets to flow through VETH devices.

When there are VLANs on a VETH device, the packets being transmitted
through the VETH device may be 4 bytes bigger than MTU.  A check
in dev_forward_skb did not take this into account and so dropped
these packets.

This patch is needed at least as far back as 2.6.34.7 and should
be considered for -stable.
Signed-off-by: NBen Greear <greearb@candelatech.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -1485,7 +1485,7 @@ int dev_forward_skb(struct net_device *dev, struct sk_buff *skb)
 	nf_reset(skb);
 
 	if (!(dev->flags & IFF_UP) ||
-	    (skb->len > (dev->mtu + dev->hard_header_len))) {
+	    (skb->len > (dev->mtu + dev->hard_header_len + VLAN_HLEN))) {
 		kfree_skb(skb);
 		return NET_RX_DROP;
 	}