Handle IPv6 forwarding with netfilter

[troglobit]

Compared to /proc/sys/net/ipv4/conf/forwarding, /proc/sys/net/ipv6/conf/forwarding is not working in the same way. For details, see https://docs.kernel.org/networking/ip-sysctl.html

/proc/sys/net/ipv6/conf/all/forwarding is enabled by default and can only be overridden per interface with a netfilter rule: disabling forwarding of IPv6 traffic on ingress, when yang ipv6 forwarding is disabled.