sched: fix prev_stime calculation

Srivatsa Vaddagiri noticed occasionally incorrect CPU usage
values in top and tracked it down to stime going below 0 in
task_stime(). Negative values are possible there due to the
sampled nature of stime/utime.

Fix suggested by Balbir Singh.
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Tested-by: NSrivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Reviewed-by: NBalbir Singh <balbir@linux.vnet.ibm.com>

fs/proc/array.c

@@ -374,7 +374,9 @@ static cputime_t task_stime(struct task_struct *p)
 	stime = nsec_to_clock_t(p->se.sum_exec_runtime) -
 			cputime_to_clock_t(task_utime(p));
 
+	if (stime >= 0)
 		p->prev_stime = max(p->prev_stime, clock_t_to_cputime(stime));
+
 	return p->prev_stime;
 }
 #endif