fq_codel: fix return value of fq_codel_drop()

The ->drop() is supposed to return the number of bytes it dropped,
however fq_codel_drop() returns the index of the flow where it drops
a packet from.

Fix this by introducing a helper to wrap fq_codel_drop().

Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Cong Wang <cwang@twopensource.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_fq_codel.c

@@ -163,6 +163,15 @@ static unsigned int fq_codel_drop(struct Qdisc *sch)
 	return idx;
 }
 
+static unsigned int fq_codel_qdisc_drop(struct Qdisc *sch)
+{
+	unsigned int prev_backlog;
+
+	prev_backlog = sch->qstats.backlog;
+	fq_codel_drop(sch);
+	return prev_backlog - sch->qstats.backlog;
+}
+
 static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 {
 	struct fq_codel_sched_data *q = qdisc_priv(sch);
@@ -604,7 +613,7 @@ static struct Qdisc_ops fq_codel_qdisc_ops __read_mostly = {
 	.enqueue	=	fq_codel_enqueue,
 	.dequeue	=	fq_codel_dequeue,
 	.peek		=	qdisc_peek_dequeued,
-	.drop		=	fq_codel_drop,
+	.drop		=	fq_codel_qdisc_drop,
 	.init		=	fq_codel_init,
 	.reset		=	fq_codel_reset,
 	.destroy	=	fq_codel_destroy,