iio: buffer: re-introduce bitmap_zalloc() for trialmask

Commit 3862828a ("iio: buffer: Switch to bitmap_zalloc()") introduced
bitmap_alloc(), but commit 20ea39ef ("iio: Fix scan mask selection")
reverted it.

This change adds it back. The only difference is that it's adding
bitmap_zalloc(). There might be some changes later that would require
initializing it to zero. In any case, now it's already zero-ing the
trialmask.

Appears to have been the result of merge conflict resolution rather
than an intentional revert.
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-buffer.c

@@ -316,8 +316,7 @@ static int iio_scan_mask_set(struct iio_dev *indio_dev,
 	const unsigned long *mask;
 	unsigned long *trialmask;
 
-	trialmask = kcalloc(BITS_TO_LONGS(indio_dev->masklength),
-			    sizeof(*trialmask), GFP_KERNEL);
+	trialmask = bitmap_zalloc(indio_dev->masklength, GFP_KERNEL);
 	if (trialmask == NULL)
 		return -ENOMEM;
 	if (!indio_dev->masklength) {