fix: IPv6-hbb: discard as indicated by hbh type

Discard the packet if the Hop-by-Hop header type is 0b11 and the packet
destination address is multicast.

src/iface/interface/ipv6.rs

@@ -291,6 +291,8 @@ impl InterfaceInner {
                         Ipv6OptionFailureType::DiscardSendUnicast => {
                             if !ipv6_repr.dst_addr.is_multicast() {
                                 return HopByHopResponse::Discard(param_problem());
+                            } else {
+                                return HopByHopResponse::Discard(None);
                             }
                         }
                     }