mac80211: Use rcu_barrier() on unload.

The mac80211 module uses rcu_call() thus it should use rcu_barrier()
on module unload.

The rcu_barrier() is placed in mech.c ieee80211_stop_mesh() which is
invoked from ieee80211_stop() in case vif.type == NL80211_IFTYPE_MESH_POINT.
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mac80211/mesh.c

@@ -494,7 +494,7 @@ void ieee80211_stop_mesh(struct ieee80211_sub_if_data *sdata)
 	 * should it be using the interface and enqueuing
 	 * frames at this very time on another CPU.
 	 */
-	synchronize_rcu();
+	rcu_barrier(); /* Wait for RX path and call_rcu()'s */
 	skb_queue_purge(&sdata->u.mesh.skb_queue);
 }