Use fib[46]_lookup() in mtu calculations.

fib[46]_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.

Differential Revision: https://reviews.freebsd.org/D24974