[IPV6] Fix device multicast list leakage when forwarding is on.

We failed to leave all-routers multicast address.
Signed-off-by: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>

net/ipv6/mcast.c

@@ -2110,6 +2110,11 @@ void ipv6_mc_destroy_dev(struct inet6_dev *idev)
 	 */
 	__ipv6_dev_mc_dec(idev->dev, idev, &maddr);
 
+	if (idev->cnf.forwarding) {
+		ipv6_addr_all_routers(&maddr);
+		__ipv6_dev_mc_dec(idev->dev, idev, &maddr);
+	}
+
 	write_lock_bh(&idev->lock);
 	while ((i = idev->mc_list) != NULL) {
 		idev->mc_list = i->next;