Add nhop parameter to rti_filter callback.

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.

Reviewed by: ae
Differential Revision: https://reviews.freebsd.org/D24440