mlxsw: spectrum_router: Associate RIFs with correct VR

When a router interface (RIF) is created due to a netdev being enslaved
to a VRF master, then it should be associated with the appropriate
virtual router (VR) and not the default one.

If netdev is a VRF slave, lookup the VR based on the VRF's table ID.
Otherwise default to the MAIN table.

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