pids: Compute task_tgid using signal->leader_pid

The cost is the the same and this removes the need
to worry about complications that come from de_thread
and group_leader changing.

__task_pid_nr_ns has been updated to take advantage of this change.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

include/linux/sched/signal.h

@@ -564,6 +564,11 @@ struct pid *task_pid_type(struct task_struct *task, enum pid_type type)
 	return task->pids[type].pid;
 }
 
+static inline struct pid *task_tgid(struct task_struct *task)
+{
+	return task->signal->leader_pid;
+}
+
 static inline int get_nr_threads(struct task_struct *tsk)
 {
 	return tsk->signal->nr_threads;