sched: Fix sched::sched_stat_wait tracepoint field

This weird perf trace output:

  cc1-9943  [001]  2802.059479616: sched_stat_wait: task: as:9944 wait: 2801938766276 [ns]

Is caused by setting one component field of the delta to zero
a bit too early. Move it to later.

( Note, this does not affect the NEW_FAIR_SLEEPERS interactivity bug,
  it's just a reporting bug in essence. )

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Nikos Chantziaras <realnc@arcor.de>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <4AA93D34.8040500@arcor.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_fair.c

@@ -545,14 +545,13 @@ update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	schedstat_set(se->wait_count, se->wait_count + 1);
 	schedstat_set(se->wait_sum, se->wait_sum +
 			rq_of(cfs_rq)->clock - se->wait_start);
-	schedstat_set(se->wait_start, 0);
-
 #ifdef CONFIG_SCHEDSTATS
 	if (entity_is_task(se)) {
 		trace_sched_stat_wait(task_of(se),
 			rq_of(cfs_rq)->clock - se->wait_start);
 	}
 #endif
+	schedstat_set(se->wait_start, 0);
 }
 
 static inline void