openvswitch: netlink: support L3 packets

Extend the ovs flow netlink protocol to support L3 packets. Packets without
OVS_KEY_ATTR_ETHERNET attribute specify L3 packets; for those, the
OVS_KEY_ATTR_ETHERTYPE attribute is mandatory.

Push/pop vlan actions are only supported for Ethernet packets.

Based on previous versions by Lorand Jakab and Simon Horman.

Signed-off-by: Lorand Jakab <lojakab@cisco.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>