include/linux/sched.h: don't use task->pid/tgid in same_thread_group/has_group_leader_pid

task_struct->pid/tgid should go away.

1. Change same_thread_group() to use task->signal for comparison.

2. Change has_group_leader_pid(task) to compare task_pid(task) with
   signal->leader_pid.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Sergey Dyasly <dserrg@gmail.com>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -2169,15 +2169,15 @@ static inline bool thread_group_leader(struct task_struct *p)
  * all we care about is that we have a task with the appropriate
  * pid, we don't actually care if we have the right task.
  */
-static inline int has_group_leader_pid(struct task_struct *p)
+static inline bool has_group_leader_pid(struct task_struct *p)
 {
-	return p->pid == p->tgid;
+	return task_pid(p) == p->signal->leader_pid;
 }
 
 static inline
-int same_thread_group(struct task_struct *p1, struct task_struct *p2)
+bool same_thread_group(struct task_struct *p1, struct task_struct *p2)
 {
-	return p1->tgid == p2->tgid;
+	return p1->signal == p2->signal;
 }
 
 static inline struct task_struct *next_thread(const struct task_struct *p)