posix-cpu-timer: Tidy up group_leader logic in lookup_task

Replace has_group_leader_pid with thread_group_leader.  Years ago Oleg
suggested changing thread_group_leader to has_group_leader_pid to handle
races.  Looking at the code then and now I don't see how it ever helped.
Especially as then the code really did need to be the
thread_group_leader.

Today it doesn't make a difference if thread_group_leader races with
de_thread as the task returned from lookup_task in the non-thread case is
just used to find values in task->signal.

Since the races with de_thread have never been handled revert
has_group_header_pid to thread_group_leader for clarity.

Update the comment in lookup_task to remove implementation details that
are no longer true and to mention task->signal instead of task->sighand,
as the relevant cpu timer details are all in task->signal.

Ref: 55e8c8eb ("posix-cpu-timers: Store a reference to a pid not a task")
Ref: c0deae8c ("posix-cpu-timers: Rcu_read_lock/unlock protect find_task_by_vpid call")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

kernel/time/posix-cpu-timers.c

@@ -69,12 +69,8 @@ static struct task_struct *lookup_task(const pid_t pid, bool thread,
 	if (gettime) {
 		/*
 		 * For clock_gettime(PROCESS) the task does not need to be
-		 * the actual group leader. tsk->sighand gives
+		 * the actual group leader. task->signal gives
 		 * access to the group's clock.
-		 *
-		 * Timers need the group leader because they take a
-		 * reference on it and store the task pointer until the
-		 * timer is destroyed.
 		 */
 		return (p == current || thread_group_leader(p)) ? p : NULL;
 	}
@@ -82,7 +78,7 @@ static struct task_struct *lookup_task(const pid_t pid, bool thread,
 	/*
 	 * For processes require that p is group leader.
 	 */
-	return has_group_leader_pid(p) ? p : NULL;
+	return thread_group_leader(p) ? p : NULL;
 }
 
 static struct task_struct *__get_task_for_clock(const clockid_t clock,