bridge: fix bridge netlink RCU usage

When the STP timer fires, it can call br_ifinfo_notify(),
which in turn ends up in the new br_get_link_af_size().
This function is annotated to be using RTNL locking, which
clearly isn't the case here, and thus lockdep warns:

  ===============================
  [ INFO: suspicious RCU usage. ]
  3.19.0+ #569 Not tainted
  -------------------------------
  net/bridge/br_private.h:204 suspicious rcu_dereference_protected() usage!

Fix this by doing RCU locking here.

Fixes: b7853d73 ("bridge: add vlan info to bridge setlink and dellink notification messages")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_netlink.c

@@ -81,17 +81,19 @@ static size_t br_get_link_af_size_filtered(const struct net_device *dev,
 	struct net_port_vlans *pv;
 	int num_vlan_infos;
 
+	rcu_read_lock();
 	if (br_port_exists(dev))
-		pv = nbp_get_vlan_info(br_port_get_rtnl(dev));
+		pv = nbp_get_vlan_info(br_port_get_rcu(dev));
 	else if (dev->priv_flags & IFF_EBRIDGE)
 		pv = br_get_vlan_info((struct net_bridge *)netdev_priv(dev));
 	else
-		return 0;
-
-	if (!pv)
-		return 0;
-
+		pv = NULL;
+	if (pv)
 		num_vlan_infos = br_get_num_vlan_infos(pv, filter_mask);
+	else
+		num_vlan_infos = 0;
+	rcu_read_unlock();
+
 	if (!num_vlan_infos)
 		return 0;