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: Ben Greear <greearb@candelatech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>