tty: Reset itty for other pty

Commit 19ffd68f81
('pty: Remove redundant itty reset') introduced a regression
whereby the other pty's linkage is not cleared on teardown.
This triggers a false positive diagnostic in testing.

Properly reset the itty linkage.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Cc: stable <stable@vger.kernel.org> # 3.10
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -1618,6 +1618,8 @@ static void release_tty(struct tty_struct *tty, int idx)
 	tty_free_termios(tty);
 	tty_driver_remove_tty(tty->driver, tty);
 	tty->port->itty = NULL;
+	if (tty->link)
+		tty->link->port->itty = NULL;
 	cancel_work_sync(&tty->port->buf.work);
 
 	if (tty->link)