[PKT_SCHED]: Qdisc drop operation is optional

The drop operation is optional and qdiscs must check if childs support it.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_netem.c

@@ -252,9 +252,9 @@ static int netem_requeue(struct sk_buff *skb, struct Qdisc *sch)
 static unsigned int netem_drop(struct Qdisc* sch)
 {
 	struct netem_sched_data *q = qdisc_priv(sch);
-	unsigned int len;
+	unsigned int len = 0;
 
-	if ((len = q->qdisc->ops->drop(q->qdisc)) != 0) {
+	if (q->qdisc->ops->drop && (len = q->qdisc->ops->drop(q->qdisc)) != 0) {
 		sch->q.qlen--;
 		sch->qstats.drops++;
 	}

net/sched/sch_prio.c

@@ -165,7 +165,7 @@ static unsigned int prio_drop(struct Qdisc* sch)
 
 	for (prio = q->bands-1; prio >= 0; prio--) {
 		qdisc = q->queues[prio];
-		if ((len = qdisc->ops->drop(qdisc)) != 0) {
+		if (qdisc->ops->drop && (len = qdisc->ops->drop(qdisc)) != 0) {
 			sch->q.qlen--;
 			return len;
 		}

net/sched/sch_tbf.c

@@ -177,9 +177,9 @@ static int tbf_requeue(struct sk_buff *skb, struct Qdisc* sch)
 static unsigned int tbf_drop(struct Qdisc* sch)
 {
 	struct tbf_sched_data *q = qdisc_priv(sch);
-	unsigned int len;
+	unsigned int len = 0;
 
-	if ((len = q->qdisc->ops->drop(q->qdisc)) != 0) {
+	if (q->qdisc->ops->drop && (len = q->qdisc->ops->drop(q->qdisc)) != 0) {
 		sch->q.qlen--;
 		sch->qstats.drops++;
 	}