[IPV4,IPV6] set idev/rt6i_idev to loopback instead of NULL, to omit checking if it is non-NULL.

(Based on hint by David S. Miller <davem@redhat.com>)
Signed-off-by: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>

net/ipv4/route.c

@@ -1342,10 +1342,13 @@ static void ipv4_dst_ifdown(struct dst_entry *dst, int how)
 {
 	struct rtable *rt = (struct rtable *) dst;
 	struct in_device *idev = rt->idev;
-	if (idev) {
-		rt->idev = NULL;
+	if (idev && idev->dev != &loopback_dev) {
+		struct in_device *loopback_idev = in_dev_get(&loopback_dev);
+		if (loopback_idev) {
+			rt->idev = loopback_idev;
 			in_dev_put(idev);
 		}
+	}
 }
 
 static void ipv4_link_failure(struct sk_buff *skb)

net/ipv6/route.c

@@ -155,7 +155,16 @@ static void ip6_dst_destroy(struct dst_entry *dst)
 
 static void ip6_dst_ifdown(struct dst_entry *dst, int how)
 {
-	ip6_dst_destroy(dst);
+	struct rt6_info *rt = (struct rt6_info *)dst;
+	struct inet6_dev *idev = rt->rt6i_idev;
+
+	if (idev != NULL && idev->dev != &loopback_dev) {
+		struct inet6_dev *loopback_idev = in6_dev_get(&loopback_dev);
+		if (loopback_idev != NULL) {
+			rt->rt6i_idev = loopback_idev;
+			in6_dev_put(idev);
+		}
+	}
 }
 
 /*