tty: Make sure we're opening unpaired pty

pty_open_unpaired_slave must be called for
pty peers only and it caused no problems so
far but better to be sure it's pty peer we're
restoring (there gonna be more drivers with
time).
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

tty.c

@@ -957,7 +957,7 @@ static int tty_open(struct file_desc *d)
 	if (!info->create)
 		return receive_tty(info);
 
-	if (!tty_is_master(info))
+	if (is_pty(info->driver) && !tty_is_master(info))
 		return pty_open_unpaired_slave(d, info);
 
 	return info->driver->open(info);