sch_sfb: Also store skb len before calling child enqueue

Cong Wang noticed that the previous fix for sch_sfb accessing the queued
skb after enqueueing it to a child qdisc was incomplete: the SFB enqueue
function was also calling qdisc_qstats_backlog_inc() after enqueue, which
reads the pkt len from the skb cb field. Fix this by also storing the skb
len, and using the stored value to increment the backlog after enqueueing.

Fixes: 9efd23297c ("sch_sfb: Don't assume the skb is still around after enqueueing to child")
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Acked-by: Cong Wang <cong.wang@bytedance.com>
Link: https://lore.kernel.org/r/20220905192137.965549-1-toke@toke.dk
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/sched/sch_sfb.c

@@ -281,6 +281,7 @@ static int sfb_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 {
 
 	struct sfb_sched_data *q = qdisc_priv(sch);
+	unsigned int len = qdisc_pkt_len(skb);
 	struct Qdisc *child = q->qdisc;
 	struct tcf_proto *fl;
 	struct sfb_skb_cb cb;
@@ -403,7 +404,7 @@ enqueue:
 	memcpy(&cb, sfb_skb_cb(skb), sizeof(cb));
 	ret = qdisc_enqueue(skb, child, to_free);
 	if (likely(ret == NET_XMIT_SUCCESS)) {
-		qdisc_qstats_backlog_inc(sch, skb);
+		sch->qstats.backlog += len;
 		sch->q.qlen++;
 		increment_qlen(&cb, q);
 	} else if (net_xmit_drop_count(ret)) {