sched/numa: Fix math underflow in task_tick_numa()

The NUMA balancing code implements delays in scanning by
advancing curr->node_stamp beyond curr->se.sum_exec_runtime.

With unsigned math, that creates an underflow, which results
in task_numa_work being queued all the time, even when we
don't want to.

Avoiding the math underflow makes it possible to reduce CPU
overhead in the NUMA balancing code.

Reported-and-tested-by: Jan Stancek <[e-mail]>
Signed-off-by: Rik van Riel <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/sched/fair.c

···
 	now = curr->se.sum_exec_runtime;
 	period = (u64)curr->numa_scan_period * NSEC_PER_MSEC;
 
-	if (now - curr->node_stamp > period) {
+	if (now > curr->node_stamp + period) {
 		if (!curr->node_stamp)
 			curr->numa_scan_period = task_scan_min(curr);
 		curr->node_stamp += period;