net: sched: pie: avoid slow division in drop probability decay

As per RFC 8033, it is sufficient for the drop probability
decay factor to have a value of (1 - 1/64) instead of 98%.
This avoids the need to do slow division.
Suggested-by: David Laight <David.Laight@aculab.com>
Signed-off-by: Leslie Monis <lesliemonis@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_pie.c

@@ -429,7 +429,8 @@ static void calculate_probability(struct Qdisc *sch)
 	 */
 
 	if (qdelay == 0 && qdelay_old == 0 && update_prob)
-		q->vars.prob = 98 * div_u64(q->vars.prob, 100);
+		/* Reduce drop probability to 98.4% */
+		q->vars.prob -= q->vars.prob / 64u;
 
 	q->vars.qdelay = qdelay;
 	q->vars.qlen_old = qlen;