routing: retire rib_lookup_info()

This function was added in pre-epoch era ( 9a1b64d5a0224 ) to
provide public rtentry access interface & hide rtentry internals.
The implementation is based on the large on-stack copying and
refcounting of the referenced objects (ifa/ifp).
It has become obsolete after epoch & nexthop introduction. Convert
the last remaining user and remove the function itself.

Differential Revision: https://reviews.freebsd.org/D36197