dma-buf: Prettify typecasts for dma-fence-chain

Inside dma-fence-chain, we use a cmpxchg on an RCU-protected pointer. To
avoid the sparse warning for using the RCU pointer directly, we have to
cast away the __rcu annotation. However, we don't need to use void*
everywhere and can stick to the dma_fence*.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200409110101.18400-1-chris@chris-wilson.co.uk

drivers/dma-buf/dma-fence-chain.c

@@ -62,7 +62,8 @@ struct dma_fence *dma_fence_chain_walk(struct dma_fence *fence)
 			replacement = NULL;
 		}
 
-		tmp = cmpxchg((void **)&chain->prev, (void *)prev, (void *)replacement);
+		tmp = cmpxchg((struct dma_fence __force **)&chain->prev,
+			      prev, replacement);
 		if (tmp == prev)
 			dma_fence_put(tmp);
 		else