bridge: Fix IGMP3 report parsing

The IGMP3 report parsing is looking at the wrong address for
group records.  This patch fixes it.
Reported-by: Banyeer <banyeer@yahoo.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_multicast.c

@@ -723,7 +723,7 @@ static int br_multicast_igmp3_report(struct net_bridge *br,
 		if (!pskb_may_pull(skb, len))
 			return -EINVAL;
 
-		grec = (void *)(skb->data + len);
+		grec = (void *)(skb->data + len - sizeof(*grec));
 		group = grec->grec_mca;
 		type = grec->grec_type;