[PATCH] USB: fix race between disconnect and write of acm driver

acm uses a workqueue to defer part of a write operation.
In case of disconnect this work must be waited for.
  - fix race between write and disconnect
Signed-off-by: Oliver Neukum <oliver@neukum.name>
Signed-off-by: Vojtech Pavlik <vojtech@suse.cz>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/usb/class/cdc-acm.c

@@ -691,6 +691,8 @@ static void acm_disconnect(struct usb_interface *intf)
 	usb_unlink_urb(acm->readurb);
 	usb_unlink_urb(acm->writeurb);
 
+	flush_scheduled_work(); /* wait for acm_softint */
+
 	kfree(acm->ctrlurb->transfer_buffer);
 
 	usb_driver_release_interface(&acm_driver, acm->data);