Switch ip_output/icmp_reflect rt lookup calls with fib4_lookup.

fib4_lookup_nh_ represents pre-epoch generation of fib api,
providing less guarantees over pointer validness and requiring
on-stack data copying.

Conversion is straight-forwarded, as the only 2 differences are
requirement of running in network epoch and the need to handle
RTF_GATEWAY case in the caller code.

Reviewed by: ae
Differential Revision: https://reviews.freebsd.org/D24976