change next_thread() to use __next_thread() ?: group_leader

This relies on fact that group leader is always the 1st entry in the
signal->thread_head list.

With or without this change, if the lockless next_thread(last_thread)
races with exec it can return the old or the new leader.

We are almost ready to kill task->thread_group, after this change its
only user is thread_group_empty().

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Oleg Nesterov <[e-mail]>
Cc: Eric W. Biederman <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/sched/signal.h

···
 					thread_node);
 }
 
-static inline struct task_struct *next_thread(const struct task_struct *p)
+static inline struct task_struct *next_thread(struct task_struct *p)
 {
-	return list_entry_rcu(p->thread_group.next,
-			      struct task_struct, thread_group);
+	return __next_thread(p) ?: p->group_leader;
 }
 
 static inline int thread_group_empty(struct task_struct *p)