dma-buf: return only unsignaled fences in dma_fence_unwrap_for_each v3

dma_fence_chain containers cleanup signaled fences automatically, so
filter those out from arrays as well.

v2: fix missing walk over the array
v3: massively simplify the patch and actually update the description.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220518135844.3338-4-christian.koenig@amd.com

include/linux/dma-fence-unwrap.h

@@ -43,9 +43,13 @@ struct dma_fence *dma_fence_unwrap_next(struct dma_fence_unwrap *cursor);
  * Unwrap dma_fence_chain and dma_fence_array containers and deep dive into all
  * potential fences in them. If @head is just a normal fence only that one is
  * returned.
+ *
+ * Note that signalled fences are opportunistically filtered out, which
+ * means the iteration is potentially over no fence at all.
  */
 #define dma_fence_unwrap_for_each(fence, cursor, head)			\
 	for (fence = dma_fence_unwrap_first(head, cursor); fence;	\
-	     fence = dma_fence_unwrap_next(cursor))
+	     fence = dma_fence_unwrap_next(cursor))			\
+		if (!dma_fence_is_signaled(fence))
 
 #endif