skb_vlan_pop() expects skb->protocol to be a valid TPID for double
tagged frames. So set skb->protocol to the TPID and let skb_vlan_pop()
shift the true ethertype into position for us.

Fixes: 5108bbad ("openvswitch: add processing of L3 packets")
Signed-off-by: NEric Garver <e@erig.me>
Reviewed-by: NJiri Benc <jbenc@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>