mlxsw: spectrum_router: Adjust loopback RIF configuration

In Spectrum-2, the underlay routing table is pointed by an underlay router
interface in contrary to Spectrum where only an underlay virtual router
should be set. That makes the underlay virtual router field in RITR
reserved for Spectrum-2.

Change loopback RIF creation function to support the new underlay RIF
field, however leave this field reserved for Spectrum-1 updates.

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>