posix-timers: Noralize good_sigevent

In good_sigevent directly compute the default return value as
"task_tgid(current)".  This is exactly the same as
"task_pid(current->group_leader)" but written more clearly.

In the thread case first compute the thread's pid.  Then veify that
attached to that pid is a thread of the current thread group.

This has the net effect of making the code a little clearer, and
making it obvious that posix timers never look up a process by a the
pid of a thread.
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

kernel/time/posix-timers.c

@@ -433,11 +433,13 @@ static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer)
 
 static struct pid *good_sigevent(sigevent_t * event)
 {
-	struct task_struct *rtn = current->group_leader;
+	struct pid *pid = task_tgid(current);
+	struct task_struct *rtn;
 
 	switch (event->sigev_notify) {
 	case SIGEV_SIGNAL | SIGEV_THREAD_ID:
-		rtn = find_task_by_vpid(event->sigev_notify_thread_id);
+		pid = find_vpid(event->sigev_notify_thread_id);
+		rtn = pid_task(pid, PIDTYPE_PID);
 		if (!rtn || !same_thread_group(rtn, current))
 			return NULL;
 		/* FALLTHRU */
@@ -447,7 +449,7 @@ static struct pid *good_sigevent(sigevent_t * event)
 			return NULL;
 		/* FALLTHRU */
 	case SIGEV_NONE:
-		return task_pid(rtn);
+		return pid;
 	default:
 		return NULL;
 	}