ipv6: release reference of ip6_null_entry's dst entry in __ip6_del_rt

as we hold dst_entry before we call __ip6_del_rt,
so we should alse call dst_release not only return
-ENOENT when the rt6_info is ip6_null_entry.

and we already hold the dst entry, so I think it's
safe to call dst_release out of the write-read lock.
Signed-off-by: NGao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1593,17 +1593,18 @@ static int __ip6_del_rt(struct rt6_info *rt, struct nl_info *info)
 	struct fib6_table *table;
 	struct net *net = dev_net(rt->dst.dev);
 
-	if (rt == net->ipv6.ip6_null_entry)
-		return -ENOENT;
+	if (rt == net->ipv6.ip6_null_entry) {
+		err = -ENOENT;
+		goto out;
+	}
 
 	table = rt->rt6i_table;
 	write_lock_bh(&table->tb6_lock);
-
 	err = fib6_del(rt, info);
-	dst_release(&rt->dst);
-
 	write_unlock_bh(&table->tb6_lock);
 
+out:
+	dst_release(&rt->dst);
 	return err;
 }