tty: Flush tty buffers after hardware shutdown

The line discipline buffer and the tty buffers must be flushed again
after hardware shutdown; otherwise, a brief window exists between the
ldisc flush in tty_port_close_start() and the subsequent
tty_port_shutdown(), during which more data could be received into the
tty buffers. A racing open might then be able to receive data from the
previous session.
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_port.c

@@ -520,6 +520,7 @@ void tty_port_close_end(struct tty_port *port, struct tty_struct *tty)
 {
 	unsigned long flags;
 
+	tty_ldisc_flush(tty);
 	tty->closing = 0;
 
 	spin_lock_irqsave(&port->lock, flags);