staging: comedi: add detachment counter for validity checks

Add a member `detach_count` to `struct comedi_device` that is
incremented every time the device gets detached.  This will be used in
some validity checks in the 'read' and 'write' file operations to make
sure the attachment remains valid.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedidev.h

@@ -173,6 +173,7 @@ struct comedi_device {
 
 	struct device *class_dev;
 	int minor;
+	unsigned int detach_count;
 	/* hw_dev is passed to dma_alloc_coherent when allocating async buffers
 	 * for subdevices that have async_dma_dir set to something other than
 	 * DMA_NONE */

drivers/staging/comedi/drivers.c

@@ -136,6 +136,7 @@ void comedi_device_detach(struct comedi_device *dev)
 	comedi_device_cancel_all(dev);
 	down_write(&dev->attach_lock);
 	dev->attached = false;
+	dev->detach_count++;
 	if (dev->driver)
 		dev->driver->detach(dev);
 	comedi_device_detach_cleanup(dev);