userspace: add layer 3 flow and switching support

This commit relaxes the assumption that all packets have an Ethernet
header, and adds support for layer 3 flows.  For each packet received on
the Linux kernel datapath the l2 and l3 members of struct ofpbuf are
intialized appropriately, and some functions now expect this (notably
flow_extract()), in order to differentiate between layer 2 and layer 3
packets.  struct flow has now a new 'base_layer' member, because we
cannot assume that a flow has no Ethernet header when eth_src and
eth_dst are 0.  For layer 3 packets, the protocol type is still stored
in the eth_type member.

Switching L2->L3 and L3->L2 are both implemented by adding the pop_eth
and push_eth actions respectively when a transition is detected.  The
push_eth action puts 0s on both source and destination MACs.  These
addresses can be modified with mod_dl_dst and mod_dl_src actions.

Added new prerequisite MFP_ETHERNET for fields MFF_VLAN_TCI,
MFF_DL_VLAN, MFF_VLAN_VID and MFF_DL_VLAN_PCP.

Signed-off-by: Lorand Jakab <lojakab@cisco.com>
datapath: add layer 3 flow/port support

Implementation of the pop_eth and push_eth actions in the kernel, and
layer 3 flow support.

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

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

The presence of a layer 3 packet is signaled by the absence of
OVS_KEY_ATTR_ETHERNET in the packet metadata passed from user space.

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