mac80211: add missing rcu_barrier

mac80211 uses call_rcu() with functions that are
defined in the module, so it must use rcu_barrier()
at module exit time.

Luckily, this seems to not be a problem in practice
as module unload and unregistration takes a long
time and probably does multiple synchronize_rcu().
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/mac80211/main.c

@@ -1090,6 +1090,8 @@ static void __exit ieee80211_exit(void)
 		ieee80211s_stop();
 
 	ieee80211_iface_exit();
+
+	rcu_barrier();
 }