dma-buf: make fence mandatory for dma_resv_add_excl_fence v2

Calling dma_resv_add_excl_fence() with the fence as NULL and expecting
that that this frees up the fences is simply abuse of the internals of
the dma_resv object.

v2: drop the fence pruning completely.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211129120659.1815-4-christian.koenig@amd.com

drivers/dma-buf/dma-resv.c

@@ -305,8 +305,7 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
 	if (old)
 		i = old->shared_count;
 
-	if (fence)
-		dma_fence_get(fence);
+	dma_fence_get(fence);
 
 	write_seqcount_begin(&obj->seq);
 	/* write_seqcount_begin provides the necessary memory barrier */