net/flow_dissector: Save vlan ethertype from headers

Change vlan dissector key to save vlan tpid to support both 802.1Q
and 802.1AD ethertype.

Signed-off-by: Jianbo Liu <jianbol@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>