rtnetlink: rtnl_have_link_slave_info doesn't need rtnl

it can be switched to rcu.
Reviewed-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -522,11 +522,15 @@ static size_t rtnl_link_get_af_size(const struct net_device *dev,
 static bool rtnl_have_link_slave_info(const struct net_device *dev)
 {
 	struct net_device *master_dev;
+	bool ret = false;
 
-	master_dev = netdev_master_upper_dev_get((struct net_device *) dev);
+	rcu_read_lock();
+
+	master_dev = netdev_master_upper_dev_get_rcu((struct net_device *)dev);
 	if (master_dev && master_dev->rtnl_link_ops)
-		return true;
-	return false;
+		ret = true;
+	rcu_read_unlock();
+	return ret;
 }
 
 static int rtnl_link_slave_info_fill(struct sk_buff *skb,