mlxsw: spectrum_router: Use loopback RIF for unresolved nexthops

Now that the driver creates a loopback RIF during its initialization, it
can be used to program the adjacency entries for unresolved nexthops
instead of other RIFs. The loopback RIF is guaranteed to exist for the
entire life time of the driver, unlike other RIFs that come and go.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>