iio: core: annotate masklength as __private

Now that all users are using the proper accessors, we can mark
masklength as __private so that no one tries to write. We also get help
from checkers in warning us in case someone does it.

To access the private field from IIO core code, we need to use the
ACCESS_PRIVATE() macro.
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240726-dev-iio-masklength-private3-v1-23-82913fc0fb87@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-buffer.c

@@ -2269,7 +2269,7 @@ int iio_buffers_alloc_sysfs_and_mask(struct iio_dev *indio_dev)
 
 		for (i = 0; i < indio_dev->num_channels; i++)
 			ml = max(ml, channels[i].scan_index + 1);
-		indio_dev->masklength = ml;
+		ACCESS_PRIVATE(indio_dev, masklength) = ml;
 	}
 
 	if (!iio_dev_opaque->attached_buffers_cnt)

include/linux/iio/iio.h

@@ -609,7 +609,7 @@ struct iio_dev {
 	int				scan_bytes;
 
 	const unsigned long		*available_scan_masks;
-	unsigned			masklength;
+	unsigned			__private masklength;
 	const unsigned long		*active_scan_mask;
 	bool				scan_timestamp;
 	struct iio_trigger		*trig;
@@ -861,7 +861,7 @@ static inline const struct iio_scan_type
  */
 static inline unsigned int iio_get_masklength(const struct iio_dev *indio_dev)
 {
-	return indio_dev->masklength;
+	return ACCESS_PRIVATE(indio_dev, masklength);
 }
 
 /**