mlxsw: spectrum_router: Introduce loopback RIFs

When offloading L3 tunnels, an adjacency entry is created that loops the
packet back into the underlay router. Loopback interfaces then hold the
corresponding information and are created for IP-in-IP netdevices.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>