mlxsw: spectrum_router: Track next hops at CRIFs

Move the list of next hops from struct mlxsw_sp_rif to mlxsw_sp_crif. The
reason is that eventually, next hops for mlxsw uppers should be offloaded
and unoffloaded on demand as a netdevice becomes an upper, or stops being
one. Currently, next hops are tracked at RIFs, but RIFs do not exist when a
netdevice is not an mlxsw uppers. CRIFs are kept track of throughout the
netdevice lifetime.

Correspondingly, track at each next hop not its RIF, but its CRIF (from
which a RIF can always be deduced).

Note that now that next hops are tracked at a CRIF, it is not necessary to
move each over to a new RIF when it is necessary to edit a RIF. Therefore
drop mlxsw_sp_nexthop_rif_migrate() and have mlxsw_sp_rif_migrate_destroy()
call mlxsw_sp_nexthop_rif_update() directly.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Link: https://lore.kernel.org/r/e7c1c0a7dd13883b0f09aeda12c4fcf4d63a70e3.1687438411.git.petrm@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>