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
MFC after: 1 week
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D31680