rtnetlink: do not depend on RTNL for IFLA_QDISC output

dev->qdisc can be read using RCU protection.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/rtnetlink.c

@@ -1832,7 +1832,6 @@ static int rtnl_fill_ifinfo(struct sk_buff *skb,
 	if (tgt_netnsid >= 0 && nla_put_s32(skb, IFLA_TARGET_NETNSID, tgt_netnsid))
 		goto nla_put_failure;
 
-	qdisc = rtnl_dereference(dev->qdisc);
 	if (nla_put_string(skb, IFLA_IFNAME, dev->name) ||
 	    nla_put_u32(skb, IFLA_TXQLEN, dev->tx_queue_len) ||
 	    nla_put_u8(skb, IFLA_OPERSTATE,
@@ -1857,8 +1856,6 @@ static int rtnl_fill_ifinfo(struct sk_buff *skb,
 #endif
 	    put_master_ifindex(skb, dev) ||
 	    nla_put_u8(skb, IFLA_CARRIER, netif_carrier_ok(dev)) ||
-	    (qdisc &&
-	     nla_put_string(skb, IFLA_QDISC, qdisc->ops->id)) ||
 	    nla_put_ifalias(skb, dev) ||
 	    nla_put_u32(skb, IFLA_CARRIER_CHANGES,
 			atomic_read(&dev->carrier_up_count) +
@@ -1924,6 +1921,9 @@ static int rtnl_fill_ifinfo(struct sk_buff *skb,
 		goto nla_put_failure;
 
 	rcu_read_lock();
+	qdisc = rcu_dereference(dev->qdisc);
+	if (qdisc && nla_put_string(skb, IFLA_QDISC, qdisc->ops->id))
+		goto nla_put_failure_rcu;
 	if (rtnl_fill_link_af(skb, dev, ext_filter_mask))
 		goto nla_put_failure_rcu;
 	if (rtnl_fill_link_ifmap(skb, dev))