mlxsw: spectrum_router: Track NH ID's of group members

The core interfaces for collecting per-NH statistics are built around
nexthops even for resilient groups. Because mlxsw models each bucket as a
nexthop, the core next hop that a given bucket contributes to needs to be
looked up. In order to be able to match the two up, we need to track
nexthop ID for members of group nexthop objects. For simplicity, do it for
all nexthop objects, not just group members.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/184ceb6b154e08f5bcf116a705b0fcb01c31895c.1709901020.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>