proc: change first_tid() to use while_each_thread() rather than next_thread()

Rerwrite the main loop to use while_each_thread() instead of
next_thread().  We are going to fix or replace while_each_thread(),
next_thread() should be avoided whenever possible.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Sameer Nanda <snanda@chromium.org>
Cc: Sergey Dyasly <dserrg@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -3106,23 +3106,23 @@ static struct task_struct *first_tid(struct task_struct *leader,
 	}
 
 	/* If nr exceeds the number of threads there is nothing todo */
-	pos = NULL;
 	if (nr && nr >= get_nr_threads(leader))
-		goto out;
+		goto fail;
 	/* It could be unhashed before we take rcu lock */
 	if (!pid_alive(leader))
-		goto out;
+		goto fail;
 
 	/* If we haven't found our starting place yet start
 	 * with the leader and walk nr threads forward.
 	 */
-	for (pos = leader; nr > 0; --nr) {
-		pos = next_thread(pos);
-		if (pos == leader) {
+	pos = leader;
+	do {
+		if (nr-- <= 0)
+			goto found;
+	} while_each_thread(leader, pos);
+fail:
 	pos = NULL;
 	goto out;
-		}
-	}
 found:
 	get_task_struct(pos);
 out: