mlxsw: spectrum_router: Explicitly Associate RIFs with VRs

Up until now we implicitly associated all the router interfaces (RIFs)
with the first virtual router (VR). This must be changed in order to
enable VRF offload. Otherwise, a packet received via a VRF slave would
do a FIB lookup in the same table used by other VRFs.

Instead, bind the RIF to a VR according to the table where FIB lookup
should be performed for packets received via the RIF.

Currently, we only care about the MAIN and LOCAL tables (which we squash
together).

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>