pid: use pid_has_task() in __change_pid()

Replace hlist_empty() with the new pid_has_task() helper which is more
idiomatic, easier to grep for, and unifies how callers perform this
check.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20191017101832.5985-3-christian.brauner@ubuntu.com

kernel/pid.c

@@ -299,7 +299,7 @@ static void __change_pid(struct task_struct *task, enum pid_type type,
 	*pid_ptr = new;
 
 	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
-		if (!hlist_empty(&pid->tasks[tmp]))
+		if (pid_has_task(pid, tmp))
 			return;
 
 	free_pid(pid);