iio: buffer: Fix double-free in iio_buffers_alloc_sysfs_and_mask()

When __iio_buffer_alloc_sysfs_and_mask() failed, 'unwind_idx' should be
set to 'i - 1' to prevent double-free when cleanup resources.

BUG: KASAN: double-free or invalid-free in __iio_buffer_free_sysfs_and_mask+0x32/0xb0 [industrialio]
Call Trace:
 kfree+0x117/0x4c0
 __iio_buffer_free_sysfs_and_mask+0x32/0xb0 [industrialio]
 iio_buffers_alloc_sysfs_and_mask+0x60d/0x1570 [industrialio]
 __iio_device_register+0x483/0x1a30 [industrialio]
 ina2xx_probe+0x625/0x980 [ina2xx_adc]
Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: ee708e6b ("iio: buffer: introduce support for attaching more IIO buffers")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211013094923.2473-2-andriy.shevchenko@linux.intel.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-buffer.c

@@ -1620,7 +1620,7 @@ int iio_buffers_alloc_sysfs_and_mask(struct iio_dev *indio_dev)
 		buffer = iio_dev_opaque->attached_buffers[i];
 		ret = __iio_buffer_alloc_sysfs_and_mask(buffer, indio_dev, i);
 		if (ret) {
-			unwind_idx = i;
+			unwind_idx = i - 1;
 			goto error_unwind_sysfs_and_mask;
 		}
 	}