iio: buffer: Save a few cycles in 'iio_scan_mask_set()'

Use 'bitmap_alloc()' instead of 'bitmap_zalloc()' because the bitmap
is fully overridden by a 'bitmap_copy()' call just after its allocation.

While at it, fix the style of a NULL check.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/89d8a766eb971eda1ee362444a8711037bdb208c.1626261211.git.christophe.jaillet@wanadoo.frSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-buffer.c

@@ -354,8 +354,8 @@ static int iio_scan_mask_set(struct iio_dev *indio_dev,
 	const unsigned long *mask;
 	unsigned long *trialmask;
 
-	trialmask = bitmap_zalloc(indio_dev->masklength, GFP_KERNEL);
-	if (trialmask == NULL)
+	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");