staging: comedi: comedi_buf: clarify comedi_buf_write_free()

Use the helper comedi_buf_write_n_allocated() to clarify the check
to make sure the number of bytes to free is not more than the number
of bytes allocated.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_buf.c

@@ -252,11 +252,12 @@ static unsigned int comedi_buf_munge(struct comedi_async *async,
 /* transfers a chunk from writer to filled buffer space */
 unsigned comedi_buf_write_free(struct comedi_async *async, unsigned int nbytes)
 {
-	if ((int)(async->buf_write_count + nbytes -
-		  async->buf_write_alloc_count) > 0) {
+	unsigned int allocated = comedi_buf_write_n_allocated(async);
+
+	if (nbytes > allocated) {
 		dev_info(async->subdevice->device->class_dev,
 			 "attempted to write-free more bytes than have been write-allocated.\n");
-		nbytes = async->buf_write_alloc_count - async->buf_write_count;
+		nbytes = allocated;
 	}
 	async->buf_write_count += nbytes;
 	async->buf_write_ptr += nbytes;