mlxsw: spectrum_router: Split nexthop finalization to two stages

mlxsw: spectrum_router: Split nexthop finalization to two stages

Nexthop finalization consists of two steps: the part where the offload is
removed, because the backing RIF is now gone; and the part where the
association to the RIF is severed.

Extract from mlxsw_sp_nexthop_type_fini() a helper that covers the
unoffloading part, mlxsw_sp_nexthop_type_rif_gone(), so that it can later
be called independently.

Note that this swaps around the ordering of mlxsw_sp_nexthop_ipip_fini()
vs. mlxsw_sp_nexthop_rif_fini(). The current ordering is more of a
historical happenstance than a conscious decision. The two cleanups do not
depend on each other, and this change should have no observable effects.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Link: https://lore.kernel.org/r/7134559534c5f5c4807c3a1569fae56f8887e763.1687438411.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>