serial: core: Fix x_char race

The UART driver is expected to clear port->x_char after
transmission while holding the port->lock. However, the serial
core fails to take the port->lock before assigning port->xchar.
This allows for the following race

CPU 0                         |  CPU 1
                              |
                              | serial8250_handle_irq
                              |   ...
                              |   serial8250_tx_chars
                              |     if (port->x_char)
                              |       serial_out(up, UART_TX, port->x_char)
uart_send_xchar               |
  port->x_char = ch           |
                              |       port->x_char = 0
  port->ops->start_tx()       |
                              |

The x_char on CPU 0 will never be sent.

Take the port->lock in uart_send_xchar() before assigning port->x_char.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>