iio: buffer: Make IIO DMA fence release RCU-safe

The `dma_fence` documentation states that if a custom release
implementation is provided, the `dma_fence` object must be freed in an
RCU-safe way. The current `iio_dma_fence` implementation uses `kfree()`,
which might result in a use-after-free.

Remove the custom `release` implementation. This makes the DMA fence core
fall back to `dma_fence_free()`, which calls `kfree_rcu()` on the fence.
This requires that the fence be the first member of `struct iio_dma_fence`.

Using the default release method for extended DMA fence structures is a
common pattern.

Reported-by: codex:gpt-5.6
Fixes: 3e26d9f08f ("iio: core: Add new DMABUF interface infrastructure")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>

drivers/iio/industrialio-buffer.c

@@ -54,6 +54,10 @@ struct iio_dmabuf_priv {
 };
 
 struct iio_dma_fence {
+	/*
+	 * Must remain the first member so the default release callback can pass
+	 * the fence directly to dma_fence_free().
+	 */
 	struct dma_fence base;
 	spinlock_t lock; /* protects base */
 	struct iio_dmabuf_priv *priv;
@@ -1828,18 +1832,9 @@ iio_buffer_dma_fence_get_driver_name(struct dma_fence *fence)
 	return "iio";
 }
 
-static void iio_buffer_dma_fence_release(struct dma_fence *fence)
-{
-	struct iio_dma_fence *iio_fence =
-		container_of(fence, struct iio_dma_fence, base);
-
-	kfree(iio_fence);
-}
-
 static const struct dma_fence_ops iio_buffer_dma_fence_ops = {
 	.get_driver_name	= iio_buffer_dma_fence_get_driver_name,
 	.get_timeline_name	= iio_buffer_dma_fence_get_driver_name,
-	.release		= iio_buffer_dma_fence_release,
 };
 
 static int iio_buffer_enqueue_dmabuf(struct iio_dev_buffer_pair *ib,