wgengine/filter: omit logging for all v6 multicast, remove debug panic :(

(cherry picked from commit da3b50ad88)

wgengine/filter/filter.go

@@ -349,7 +349,10 @@ func omitDropLogging(p *packet.ParsedPacket, dir direction) bool {
 			if string(dst) == ipv6AllMLDv2CapableRouters {
 				return true
 			}
-			panic(fmt.Sprintf("Got proto=%2x; src=%x dst=%x", int(p.IPProto), src, dst))
+			// Actually, just catch all multicast.
+			if dst[0] == 0xff {
+				return true
+			}
 		}
 	}
 	return false

wgengine/filter/filter_test.go

@@ -343,6 +343,12 @@ func TestOmitDropLogging(t *testing.T) {
 			dir:  out,
 			want: true,
 		},
+		{
+			name: "v6_udp_multicast",
+			pkt:  parseHexPkt(t, "60 00 00 00 00 00 11 00  fe800000000000007dc6bc04499262a3 ff120000000000000000000000008384"),
+			dir:  out,
+			want: true,
+		},
 	}
 
 	for _, tt := range tests {