userspace: add layer 3 support to packet metadata

This is needed for sending a packet back to the datapath after a miss upcall
was processed.  The presence of a layer 2 packet is signaled by adding
OVS_KEY_ATTR_ETHERNET to the packet metadata sent with the ovs_packet netlink
message.  Layer 3 packets need to be accompanied by OVS_KEY_ATTR_ETHERTYPE to
indicate network protocol.

Signed-off-by: Lorand Jakab <lojakab@cisco.com>
datapath: add layer 3 support to ovs_packet_cmd_execute()

Packet metadata for layer 3 packets is expected to contain
OVS_KEY_ATTR_ETHERTYPE to indicate the network protocol.

Signed-off-by: Lorand Jakab <lojakab@cisco.com>