net/sched: qfq: Use cl_is_active to determine whether class is active in qfq_rm_from_ag

This is more of a preventive patch to make the code more consistent and
to prevent possible exploits that employ child qlen manipulations on qfq.
use cl_is_active instead of relying on the child qdisc's qlen to determine
class activation.

Fixes: 462dbc9101acd ("pkt_sched: QFQ Plus: fair-queueing service at DRR cost")
Signed-off-by: Jamal Hadi Salim <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/sched/sch_qfq.c

···
 /* Deschedule class and remove it from its parent aggregate. */
 static void qfq_deact_rm_from_agg(struct qfq_sched *q, struct qfq_class *cl)
 {
-	if (cl->qdisc->q.qlen > 0) /* class is active */
+	if (cl_is_active(cl)) /* class is active */
 		qfq_deactivate_class(q, cl);
 
 	qfq_rm_from_agg(q, cl);