iio: buffer: document that buffer callback must be context safe

Document that the callback registered with iio_channel_get_all_cb()
must be safe to call from any context since it is called from by
iio_push_to_buffer() which can be called in any context.

Signed-off-by: David Lechner <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/buffer/industrialio-buffer-cb.c

···
 
 struct iio_cb_buffer {
 	struct iio_buffer buffer;
+	/* Must be safe to call from any context (e.g. must not sleep). */
 	int (*cb)(const void *data, void *private);
 	void *private;
 	struct iio_channel *channels;

include/linux/iio/consumer.h

···
 /**
  * iio_channel_get_all_cb() - register callback for triggered capture
  * @dev:		Pointer to client device.
- * @cb:			Callback function.
+ * @cb:			Callback function. Must be safe to call from any context
+ *			(e.g. must not sleep).
  * @private:		Private data passed to callback.
  *
  * NB right now we have no ability to mux data from multiple devices.