USB: serial: fix missing locking on fifo in write callback

On errors the fifo was reset without any locking. This could race with
write which do kfifo_put and perhaps also chars_in_buffer and write_room.

Every other access to the fifo is protected using the port lock so
better add it to the error path as well.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/generic.c

@@ -519,11 +519,14 @@ void usb_serial_generic_write_bulk_callback(struct urb *urb)
 		port->write_urb_busy = 0;
 		spin_unlock_irqrestore(&port->lock, flags);
 
-		if (status)
+		if (status) {
+			spin_lock_irqsave(&port->lock, flags);
 			kfifo_reset_out(&port->write_fifo);
-		else
+			spin_unlock_irqrestore(&port->lock, flags);
+		} else {
 			usb_serial_generic_write_start(port);
 		}
+	}
 
 	if (status)
 		dbg("%s - non-zero urb status: %d", __func__, status);