dma-fence: add comment for WARN_ON in dma_fence_release()

In dma_fence_release() there is a WARN_ON which could be triggered by
several cases of wrong dma-fence usage. This patch adds a comment to
explain two use-cases to help driver developers that use dma-fence
and trigger that WARN_ON to better understand the reasons for it.

v2: change to a more generic, one-liner comment
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>

drivers/dma-buf/dma-fence.c

@@ -171,6 +171,7 @@ void dma_fence_release(struct kref *kref)
 
 	trace_dma_fence_destroy(fence);
 
+	/* Failed to signal before release, could be a refcounting issue */
 	WARN_ON(!list_empty(&fence->cb_list));
 
 	if (fence->ops->release)