cpuacct: reduce one NULL check in fast-path

Impact: micro-optimization

In cpuacct_charge(), task_ca() will never return NULL, so change
for(...) to do { } while(...) to save one NULL check.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
LKML-Reference: <49B863F5.2060400@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -9599,10 +9599,11 @@ static void cpuacct_charge(struct task_struct *tsk, u64 cputime)
 	cpu = task_cpu(tsk);
 	ca = task_ca(tsk);
 
-	for (; ca; ca = ca->parent) {
+	do {
 		u64 *cpuusage = per_cpu_ptr(ca->cpuusage, cpu);
 		*cpuusage += cputime;
-	}
+		ca = ca->parent;
+	} while (ca);
 }
 
 struct cgroup_subsys cpuacct_subsys = {