batman-adv: fix global protection fault during soft_iface destruction

batadv_mesh_free() schedules some RCU callbacks which need the bat_priv struct
to do their jobs, while free_netdev(), which is called immediately after, is
destroying the private data.

Put an rcu_barrier() in the middle so that free_netdev() is invoked only after
all the callbacks returned.

This bug has been introduced by ab8f433dd39be94e8617cff2dfe9f7eca162eb15
("batman-adv: Move deinitialization of soft-interface to destructor")
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/batman-adv/soft-interface.c

@@ -582,6 +582,13 @@ static void batadv_softif_free(struct net_device *dev)
 {
 	batadv_debugfs_del_meshif(dev);
 	batadv_mesh_free(dev);
+
+	/* some scheduled RCU callbacks need the bat_priv struct to accomplish
+	 * their tasks. Wait for them all to be finished before freeing the
+	 * netdev and its private data (bat_priv)
+	 */
+	rcu_barrier();
+
 	free_netdev(dev);
 }