net_sched: fix a use-after-free in sfq

Fixes: 25331d6 ("net: sched: implement qstat helper routines")
Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_sfq.c

@@ -306,10 +306,10 @@ static unsigned int sfq_drop(struct Qdisc *sch)
 		len = qdisc_pkt_len(skb);
 		slot->backlog -= len;
 		sfq_dec(q, x);
-		kfree_skb(skb);
 		sch->q.qlen--;
 		qdisc_qstats_drop(sch);
 		qdisc_qstats_backlog_dec(sch, skb);
+		kfree_skb(skb);
 		return len;
 	}