Rework part of routing code to reduce difference to D26449.
* Split rt_setmetrics into `get_info_weight()` and `rt_set_expire_info()`, as these two can be applied at different entities and at different times.
* start filling rt_weight route weight in route change notifications
* Pass flowid to UDP/raw IP route lookups
* Rework `nd6_subscription_cb()` and `sysctl_dumpentry()` to prepare for the fact that rtentry can contain multiple nexthops