mlxsw: spectrum_router: Enable resolution of nexthop groups from nexthop objects

If the FIB info (i.e, 'struct fib_info', 'struct fib6_info') uses a
nexthop object, then use the object's identifier to resolve the nexthop
group.

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