[patch] sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression

[Mike Galbraith <umgwanakikbuti@gmail.com>]

Hi Peter,

While running ltp, the fates decided it was time for me to encounter
the roughly 1 out of every 10 call failure below.  As much as I run
ltp, I'm a bit surprised that I (or anyone else) haven't met this
before, but then the fates are known to be a tad fickle.

getrusage04    0  TINFO  :  Expected timers granularity is 4000 us
getrusage04    0  TINFO  :  Using 1 as multiply factor for max [us]time increment (1000+4000us)!
getrusage04    0  TINFO  :  utime:           0us; stime:         179us
getrusage04    0  TINFO  :  utime:        3751us; stime:           0us
getrusage04    1  TFAIL  :  getrusage04.c:133: stime increased > 5000us:

When applying the full rtime to either stime or utime, do not overwrite
the previously tallied value.

Fixes: 9d7fb0427648 ("sched/cputime: Guarantee stime + utime == rtime")
Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: stable@vger.kernel.org # 4.3+
---
 kernel/sched/cputime.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -608,11 +608,13 @@ static void cputime_adjust(struct task_c
 
 	if (utime == 0) {
 		stime = rtime;
+		utime = prev->utime;
 		goto update;
 	}
 
 	if (stime == 0) {
 		utime = rtime;
+		stime = prev->stime;
 		goto update;
 	}