Use light-weight versions of routing lookup functions in ng_netflow.

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