net/flow_dissector: Add support for QinQ dissection

Dissect the QinQ packets to get both outer and inner vlan information,
then store to the extended flow keys.

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