ng_nat: Pass IPv6 packets through.

ng_nat implements NAT for IPv4 traffic only. When connected to an
ng_ether node it erroneously handled IPv6 packets as well.

This change is not sufficient: ng_nat does not do any validation of IP
packets in this mode, even though they have not yet passed through
ip_input().

PR: 243096
Reported by: Robert James Hernandez <rob@sarcasticadmin.com>
Reviewed by: julian
Differential Revision: https://reviews.freebsd.org/D23080