ipv6: recursive check rt->dst.from when call rt6_check_expired

If dst cache dst_a copies from dst_b, and dst_b copies from dst_c, check
if dst_a is expired or not, we should not end with dst_a->dst.from, dst_b,
we should check dst_c.

CC: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: NLi RongQing <roy.qing.li@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -369,15 +369,11 @@ static void ip6_dst_ifdown(struct dst_entry *dst, struct net_device *dev,
 
 static bool rt6_check_expired(const struct rt6_info *rt)
 {
-	struct rt6_info *ort = NULL;
-
 	if (rt->rt6i_flags & RTF_EXPIRES) {
 		if (time_after(jiffies, rt->dst.expires))
 			return true;
 	} else if (rt->dst.from) {
-		ort = (struct rt6_info *) rt->dst.from;
-		return (ort->rt6i_flags & RTF_EXPIRES) &&
-			time_after(jiffies, ort->dst.expires);
+		return rt6_check_expired((struct rt6_info *) rt->dst.from);
 	}
 	return false;
 }