net: netem: correct the parent's backlog when corrupted packet was dropped

stable inclusion
from linux-4.19.99
commit 5e59a74d48a0003e26c08a76f208cb73e87f58ed

--------------------------------

[ Upstream commit e0ad032e ]

If packet corruption failed we jump to finish_segs and return
NET_XMIT_SUCCESS. Seeing success will make the parent qdisc
increment its backlog, that's incorrect - we need to return
NET_XMIT_DROP.

Fixes: 6071bd1a ("netem: Segment GSO packets on enqueue")
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: NSimon Horman <simon.horman@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Signed-off-by: NLi Aichun <liaichun@huawei.com>
Reviewed-by: Nguodeqing <geffrey.guo@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

net/sched/sch_netem.c

@@ -607,6 +607,8 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		}
 		/* Parent qdiscs accounted for 1 skb of size @prev_len */
 		qdisc_tree_reduce_backlog(sch, -(nb - 1), -(len - prev_len));
+	} else if (!skb) {
+		return NET_XMIT_DROP;
 	}
 	return NET_XMIT_SUCCESS;
 }