ipv6: honor ifindex in case we receive ll addresses in router advertisements

Marc Haber reported we don't honor interface indexes when we receive link
local router addresses in router advertisements. Luckily the non-strict
version of ipv6_chk_addr already does the correct job here, so we can
simply use it to lighten the checks and use those addresses by default
without any configuration change.

Link: <http://permalink.gmane.org/gmane.linux.network/391348>
Reported-by: Marc Haber <mh+netdev@zugschlus.de>
Cc: Marc Haber <mh+netdev@zugschlus.de>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ndisc.c

@@ -1183,7 +1183,7 @@ static void ndisc_router_discovery(struct sk_buff *skb)
 	 */
 	if (!in6_dev->cnf.accept_ra_from_local &&
 	    ipv6_chk_addr(dev_net(in6_dev->dev), &ipv6_hdr(skb)->saddr,
-			  NULL, 0)) {
+			  in6_dev->dev, 0)) {
 		ND_PRINTK(2, info,
 			  "RA from local address detected on dev: %s: default router ignored\n",
 			  skb->dev->name);
@@ -1337,7 +1337,7 @@ skip_linkparms:
 #ifdef CONFIG_IPV6_ROUTE_INFO
 	if (!in6_dev->cnf.accept_ra_from_local &&
 	    ipv6_chk_addr(dev_net(in6_dev->dev), &ipv6_hdr(skb)->saddr,
-			  NULL, 0)) {
+			  in6_dev->dev, 0)) {
 		ND_PRINTK(2, info,
 			  "RA from local address detected on dev: %s: router info ignored.\n",
 			  skb->dev->name);