netem: update backlog after drop

When packet is dropped from rb-tree netem the backlog statistic should
also be updated.
Reported-by: NСергеев Сергей <adron@yapic.net>
Signed-off-by: NStephen Hemminger <stephen@networkplumber.org>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_netem.c

@@ -520,6 +520,7 @@ static unsigned int netem_drop(struct Qdisc *sch)
 			skb->next = NULL;
 			skb->prev = NULL;
 			len = qdisc_pkt_len(skb);
+			sch->qstats.backlog -= len;
 			kfree_skb(skb);
 		}
 	}