ipv6: call dst_hold_safe() properly

Similar as ipv4, ipv6 path also needs to call dst_hold_safe() when
necessary to avoid double free issue on the dst.
Signed-off-by: Wei Wang <weiwan@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -5576,7 +5576,7 @@ static void __ipv6_ifa_notify(int event, struct inet6_ifaddr *ifp)
 				ip6_del_rt(rt);
 		}
 		if (ifp->rt) {
-			dst_hold(&ifp->rt->dst);
+			if (dst_hold_safe(&ifp->rt->dst))
 				ip6_del_rt(ifp->rt);
 		}
 		rt_genid_bump_ipv6(net);

net/ipv6/route.c

@@ -1366,7 +1366,7 @@ static void ip6_link_failure(struct sk_buff *skb)
 	rt = (struct rt6_info *) skb_dst(skb);
 	if (rt) {
 		if (rt->rt6i_flags & RTF_CACHE) {
-			dst_hold(&rt->dst);
+			if (dst_hold_safe(&rt->dst))
 				ip6_del_rt(rt);
 		} else if (rt->rt6i_node && (rt->rt6i_flags & RTF_DEFAULT)) {
 			rt->rt6i_node->fn_sernum = -1;