mlxsw: spectrum_router: Offload ethernet nexthops when RIF is made

As RIF is created, refresh each netxhop group tracked at the CRIF for which
the RIF was created.

Note that nothing needs to be done for IPIP nexthops. The RIF for these is
either available from the get-go, or will never be available, so no after
the fact offloading needs to be done.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>