main/nftables: don't use nexthdr to match icmpv6

https://wiki.nftables.org/wiki-nftables/index.php/Matching_packet_headers#Matching_IPv6_headers

> Caution when using ip6 nexthdr, the value only refers to the next
> header, i.e. ip6 nexthdr tcp will only match if the ipv6 packet does
> not contain any extension headers.