Switch gif(4) path verification to fib[46]_check_urfp().

fibX_lookup_nh_ represents pre-epoch generation of fib api,
providing less guarantees over pointer validness and requiring
on-stack data copying.
Use specialized fib[46]_check_urpf() from newer KPI instead,
to allow removal of older KPI.

Reviewed by: ae
Differential Revision: https://reviews.freebsd.org/D24978