Revert "TTY: pty, remove superfluous ptm test"

This reverts commit a50f724a43.

Sasha reported that this causes problems, so revert it.

Cc: Alan Cox <alan@linux.intel.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/pty.c

@@ -54,7 +54,10 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
 	wake_up_interruptible(&tty->link->write_wait);
 	if (tty->driver->subtype == PTY_TYPE_MASTER) {
 		set_bit(TTY_OTHER_CLOSED, &tty->flags);
-		devpts_pty_kill(tty->link);
+#ifdef CONFIG_UNIX98_PTYS
+		if (tty->driver == ptm_driver)
+			devpts_pty_kill(tty->link);
+#endif
 		tty_unlock();
 		tty_vhangup(tty->link);
 		tty_lock();