mlxsw: spectrum: Add definition for egress rif table

Add definition for egress router interface table. This table describes
the final part in the routing pipeline. This table matches the egress
interface index (rif index, which is set by the previous stages and
determine the out port) and makes the decision of forwarding the packet
towards the L2 logic or dropping it.

The metadata header is added to represent this internal information.
The rif index field is mapped logically to netdevice ifindex.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>