[NET_SCHED]: sch_cbq: fix cbq_undelay_prio for non-active priorites

cbq_undelay_prio is supposed to return a time delta, but returns the
current time for non-active priorities, causing cbq_undelay to mark
the priority as active and schedule a timer for twice the current
time.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_cbq.c

@@ -612,7 +612,7 @@ static unsigned long cbq_undelay_prio(struct cbq_sched_data *q, int prio)
 	unsigned long sched = now;
 
 	if (cl_prev == NULL)
-		return now;
+		return 0;
 
 	do {
 		cl = cl_prev->next_alive;