sched, cputime: Cleanups related to task_times()

- Remove if({u,s}t)s because no one call it with NULL now.
- Use cputime_{add,sub}().
- Add ifndef-endif for prev_{u,s}time since they are used
  only when !VIRT_CPU_ACCOUNTING.

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Spencer Candland <spencer@bluehost.com>
Cc: Americo Wang <xiyou.wangcong@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
LKML-Reference: <4B1624C7.7040302@jp.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/sched.c b/kernel/sched.c
index 4883fee..17e2c1d 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5184,10 +5184,8 @@
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING
 void task_times(struct task_struct *p, cputime_t *ut, cputime_t *st)
 {
-	if (ut)
-		*ut = p->utime;
-	if (st)
-		*st = p->stime;
+	*ut = p->utime;
+	*st = p->stime;
 }
 #else
 
@@ -5197,7 +5195,7 @@
 
 void task_times(struct task_struct *p, cputime_t *ut, cputime_t *st)
 {
-	cputime_t rtime, utime = p->utime, total = utime + p->stime;
+	cputime_t rtime, utime = p->utime, total = cputime_add(utime, p->stime);
 
 	/*
 	 * Use CFS's precise accounting:
@@ -5217,12 +5215,10 @@
 	 * Compare with previous values, to keep monotonicity:
 	 */
 	p->prev_utime = max(p->prev_utime, utime);
-	p->prev_stime = max(p->prev_stime, rtime - p->prev_utime);
+	p->prev_stime = max(p->prev_stime, cputime_sub(rtime, p->prev_utime));
 
-	if (ut)
-		*ut = p->prev_utime;
-	if (st)
-		*st = p->prev_stime;
+	*ut = p->prev_utime;
+	*st = p->prev_stime;
 }
 #endif