iio: buffer: Move a sanity check at the beginning of 'iio_scan_mask_set()'

This is more standard to have sanity checks at the entry of a function,
instead of allocating some memory first and having to free it if a
condition is not met.

Shuffle code a bit to check 'masklength' before calling 'bitmap_alloc()'
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/98a351adda1908c306e981b9cc86d3dbc79eb5ec.1626261211.git.christophe.jaillet@wanadoo.frSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-buffer.c

@@ -354,13 +354,14 @@ static int iio_scan_mask_set(struct iio_dev *indio_dev,
 	const unsigned long *mask;
 	unsigned long *trialmask;
 
-	trialmask = bitmap_alloc(indio_dev->masklength, GFP_KERNEL);
-	if (!trialmask)
-		return -ENOMEM;
 	if (!indio_dev->masklength) {
 		WARN(1, "Trying to set scanmask prior to registering buffer\n");
-		goto err_invalid_mask;
+		return -EINVAL;
 	}
+
+	trialmask = bitmap_alloc(indio_dev->masklength, GFP_KERNEL);
+	if (!trialmask)
+		return -ENOMEM;
 	bitmap_copy(trialmask, buffer->scan_mask, indio_dev->masklength);
 	set_bit(bit, trialmask);