ipfw_nat64: fix direct output mode

In nat64_find_route[46] handle NHF_GATEWAY flag and use destination
address from next hop to do link layer address lookup.

PR: 255928
Reviewed by: melifaro
Obtained from: Yandex LLC
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D31680

(cherry picked from commit da3a09d8941dc29f20447e263b3a6d60370c6203)