net/sched: sch_htb: warn about class in use while deleting

Add extack to warn that delete was rejected because
the class is still in use
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/sched/sch_htb.c

@@ -1709,8 +1709,10 @@ static int htb_delete(struct Qdisc *sch, unsigned long arg,
 	 * tc subsys guarantee us that in htb_destroy it holds no class
 	 * refs so that we can remove children safely there ?
 	 */
-	if (cl->children || qdisc_class_in_use(&cl->common))
+	if (cl->children || qdisc_class_in_use(&cl->common)) {
+		NL_SET_ERR_MSG(extack, "HTB class in use");
 		return -EBUSY;
+	}
 
 	if (!cl->level && htb_parent_last_child(cl))
 		last_child = 1;