net/sched: flower: Add support for matching on vlan ethertype

As flow dissector stores vlan ethertype, tc flower now can match on that.
It is to make preparation for supporting QinQ.

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