netem: update backlog after drop

When packet is dropped from rb-tree netem the backlog statistic should
also be updated.

Reported-by: Сергеев Сергей <adron@yapic.net>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index a6d788d..6b75b67 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -520,6 +520,7 @@
 			skb->next = NULL;
 			skb->prev = NULL;
 			len = qdisc_pkt_len(skb);
+			sch->qstats.backlog -= len;
 			kfree_skb(skb);
 		}
 	}