dma-buf: add WARN_ON() illegal dma-fence signaling

Calling the signaling a NULL fence is obviously a coding error in a
driver. Those functions unfortunately just returned silently without
raising a warning.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240911085903.1496-2-christian.koenig@amd.com

drivers/dma-buf/dma-fence.c

@@ -412,7 +412,7 @@ int dma_fence_signal_timestamp(struct dma_fence *fence, ktime_t timestamp)
 	unsigned long flags;
 	int ret;
 
-	if (!fence)
+	if (WARN_ON(!fence))
 		return -EINVAL;
 
 	spin_lock_irqsave(fence->lock, flags);
@@ -464,7 +464,7 @@ int dma_fence_signal(struct dma_fence *fence)
 	int ret;
 	bool tmp;
 
-	if (!fence)
+	if (WARN_ON(!fence))
 		return -EINVAL;
 
 	tmp = dma_fence_begin_signalling();