netfilter: conntrack: do not print icmpv6 as unknown via /proc

/proc/net/nf_conntrack shows icmpv6 as unknown.

Fixes: 09ec82f5 ("netfilter: conntrack: remove protocol name from l4proto struct")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_standalone.c

@@ -266,6 +266,7 @@ static const char* l4proto_name(u16 proto)
 	case IPPROTO_GRE: return "gre";
 	case IPPROTO_SCTP: return "sctp";
 	case IPPROTO_UDPLITE: return "udplite";
+	case IPPROTO_ICMPV6: return "icmpv6";
 	}
 
 	return "unknown";