sch_sfq: use del_timer_sync() in sfq_destroy()

Let's delete timer reliably in sfq_destroy().
Signed-off-by: NJarek Poplawski <jarkao2@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_sfq.c

@@ -521,7 +521,8 @@ static void sfq_destroy(struct Qdisc *sch)
 	struct sfq_sched_data *q = qdisc_priv(sch);
 
 	tcf_destroy_chain(q->filter_list);
-	del_timer(&q->perturb_timer);
+	q->perturb_period = 0;
+	del_timer_sync(&q->perturb_timer);
 }
 
 static int sfq_dump(struct Qdisc *sch, struct sk_buff *skb)