sched: debug: fix sum_exec_runtime clearing

when cleaning sched-stats also clear prev_sum_exec_runtime.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched_debug.c

@@ -283,4 +283,5 @@ void proc_sched_set_task(struct task_struct *p)
 	p->se.wait_runtime_overruns = p->se.wait_runtime_underruns = 0;
 #endif
 	p->se.sum_exec_runtime = 0;
+	p->se.prev_sum_exec_runtime	= 0;
 }