iio: buffer: correctly return bytes written in output buffers

If for some reason 'rb->access->write()' does not write the full
requested data and the O_NONBLOCK is set, we would return 'n' to
userspace which is not really truth. Hence, let's return the number of
bytes we effectively wrote.

Fixes: 9eeee3b0 ("iio: Add output buffer support")
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20230216101452.591805-2-nuno.sa@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-buffer.c

@@ -220,7 +220,7 @@ static ssize_t iio_buffer_write(struct file *filp, const char __user *buf,
 	} while (ret == 0);
 	remove_wait_queue(&rb->pollq, &wait);
 
-	return ret < 0 ? ret : n;
+	return ret < 0 ? ret : written;
 }
 
 /**