One of the goals of the new routing KPI defined in r359823 is to entirely hide`struct rtentry` from the consumers.
It will allow to improve routing subsystem internals and deliver more features much faster.
This change is one of the ongoing changes to eliminate direct struct rtentry field accesses.

Additionally, with the followup multipath changes, single
rtentry can point to multiple nexthops.

With that in mind, convert rti_filter callback used when traversing the routing table to accept
pair (rt, nhop) instead of nexthop.