dma-buf: make dma_fence structure a bit smaller v2

We clear the callback list on kref_put so that by the time we
release the fence it is unused. No one should be adding to the cb_list
that they don't themselves hold a reference for.

This small change is actually making the structure 16% smaller.

v2: add the comment to the code as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/322916/

include/linux/dma-fence.h

@@ -65,8 +65,14 @@ struct dma_fence_cb;
 struct dma_fence {
 	struct kref refcount;
 	const struct dma_fence_ops *ops;
-	struct rcu_head rcu;
-	struct list_head cb_list;
+	/* We clear the callback list on kref_put so that by the time we
+	 * release the fence it is unused. No one should be adding to the cb_list
+	 * that they don't themselves hold a reference for.
+	 */
+	union {
+		struct rcu_head rcu;
+		struct list_head cb_list;
+	};
 	spinlock_t *lock;
 	u64 context;
 	u64 seqno;