Staging: batman-adv: wait for call_rcu before unloading module

synchronize_rcu respective synchronize_net only waits for the rcu grace
period to elapse and we may fail to finish the calls which were made to
call_rcu in that time. In result the module could be unloaded during the
execution of the RCU callbacks.

rcu_barrier[1] will now wait for all outstanding RCU callbacks to finish
before continuing.

[1] Documentation/RCU/rcubarrier.txt
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/batman-adv/main.c

@@ -72,7 +72,7 @@ static void __exit batman_exit(void)
 	destroy_workqueue(bat_event_workqueue);
 	bat_event_workqueue = NULL;
 
-	synchronize_net();
+	rcu_barrier();
 }
 
 int mesh_init(struct net_device *soft_iface)