posix_cpu_timers_exit_group(): Do not use thread_group_cputimer()

When the process exits we don't have to run new cputimer nor
use running one (as it not accounts when tsk->exit_state != 0)
to get process CPU times. As there is only one thread we can
just use CPU times fields from task and signal structs.

Signed-off-by: Stanislaw Gruszka <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Roland McGrath <[e-mail]>
Cc: Vitaly Mayatskikh <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/posix-cpu-timers.c

···
 }
 void posix_cpu_timers_exit_group(struct task_struct *tsk)
 {
-	struct task_cputime cputime;
+	struct signal_struct *const sig = tsk->signal;
 
-	thread_group_cputimer(tsk, &cputime);
 	cleanup_timers(tsk->signal->cpu_timers,
-		       cputime.utime, cputime.stime, cputime.sum_exec_runtime);
+		       cputime_add(tsk->utime, sig->utime),
+		       cputime_add(tsk->stime, sig->stime),
+		       tsk->se.sum_exec_runtime + sig->sum_sched_runtime);
 }
 
 static void clear_dead_task(struct k_itimer *timer, union cpu_time_count now)