serial: core: Claim port mutex for set_ldisc()

Three UART drivers (8250, atmel & amba-pl010) enable modem status
interrupts if the line discipline is changed to N_PPS. However,
the uart port flags may only be safely modified while holding the
port mutex.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -1245,8 +1245,11 @@ static void uart_set_ldisc(struct tty_struct *tty)
 	struct uart_state *state = tty->driver_data;
 	struct uart_port *uport = state->uart_port;
 
-	if (uport->ops->set_ldisc)
+	if (uport->ops->set_ldisc) {
+		mutex_lock(&state->port.mutex);
 		uport->ops->set_ldisc(uport, tty->termios.c_line);
+		mutex_unlock(&state->port.mutex);
+	}
 }
 
 static void uart_set_termios(struct tty_struct *tty,