sched: Fix spurious load spikes

Fixes spurious system load spikes observed in /proc/loadavgrt, as described in:

  Bug 253103: /proc/loadavgrt issues weird results
  https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=253103Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com&gt;>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/sched.c

@@ -3115,6 +3115,13 @@ unsigned long nr_iowait(void)
 	for_each_possible_cpu(i)
 		sum += atomic_read(&cpu_rq(i)->nr_iowait);
 
+	/*
+	 * Since we read the counters lockless, it might be slightly
+	 * inaccurate. Do not allow it to go below zero though:
+	 */
+	if (unlikely((long)sum < 0))
+		sum = 0;
+
 	return sum;
 }