ipv6: Compare addresses only bits up to the prefix length (RFC6724).

Compare bits up to the source address's prefix length only to
allows DNS load balancing to continue to be used as a tie breaker.
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -1070,8 +1070,10 @@ static int ipv6_get_saddr_eval(struct net *net,
 		break;
 	case IPV6_SADDR_RULE_PREFIX:
 		/* Rule 8: Use longest matching prefix */
-		score->matchlen = ret = ipv6_addr_diff(&score->ifa->addr,
-						       dst->addr);
+		ret = ipv6_addr_diff(&score->ifa->addr, dst->addr);
+		if (ret > score->ifa->prefix_len)
+			ret = score->ifa->prefix_len;
+		score->matchlen = ret;
 		break;
 	default:
 		ret = 0;