mlxsw: spectrum_router: Add underlay RIF ID support

Spectrum-2 GRE tunnels underlay should be given not only the virtual router
information for an encapsulated packet lookup, but also an underlay RIF
object which belongs to a virtual router.

Therefore add ul_rif_id field in struct mlxsw_sp_rif_ipip_lb, to be used
later in Spectrum-2 underlay RIF implementation. This field complements
ul_vr_id field, already present and defined as reserved for Spectrum-2.

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