Use recently-added combination of `fib[46]_lookup_rt()` which
returns rtentry & raw nexthop with `rt_get_inet[6]_plen()` which
returns address/prefix length of prefix inside `rt`.
Add `nhop_select_func()` wrapper around inlined `nhop_select()` to
allow callers external to the routing subsystem select the proper
nexthop from the multipath group without including internal headers.
New calls does not require reference counting objects and reduce
the amount of copied/processed rtentry data.
Differential Revision: https://reviews.freebsd.org/D27675
4e19e0d92a