bridge: prevent flooding IPv6 packets that do not have a listener

Currently if there is no listener for a certain group then IPv6 packets
for that group are flooded on all ports, even though there might be no
host and router interested in it on a port.

With this commit they are only forwarded to ports with a multicast
router.

Just like commit bd4265fe ("bridge: Only flood unregistered groups
to routers") did for IPv4, let's do the same for IPv6 with the same
reasoning.
Signed-off-by: NLinus Lüssing <linus.luessing@web.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_multicast.c

@@ -1491,8 +1491,14 @@ static int br_multicast_ipv6_rcv(struct net_bridge *br,
 	 *  - MLD has always Router Alert hop-by-hop option
 	 *  - But we do not support jumbrograms.
 	 */
-	if (ip6h->version != 6 ||
-	    ip6h->nexthdr != IPPROTO_HOPOPTS ||
+	if (ip6h->version != 6)
+		return 0;
+
+	/* Prevent flooding this packet if there is no listener present */
+	if (ipv6_is_transient_multicast(&ip6h->daddr))
+		BR_INPUT_SKB_CB(skb)->mrouters_only = 1;
+
+	if (ip6h->nexthdr != IPPROTO_HOPOPTS ||
 	    ip6h->payload_len == 0)
 		return 0;