sched: remove do_div() from __sched_slice()

Yanmin Zhang noticed a nice optimization:

  p = l * nr / nl, nl = l/g -> p = g * nr

which eliminates a do_div() from __sched_period().
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_fair.c

@@ -248,8 +248,8 @@ static u64 __sched_period(unsigned long nr_running)
 	unsigned long nr_latency = sched_nr_latency;
 
 	if (unlikely(nr_running > nr_latency)) {
+		period = sysctl_sched_min_granularity;
 		period *= nr_running;
-		do_div(period, nr_latency);
 	}
 
 	return period;