dma-buf: Use atomic_fetch_add() for the context id

Now that atomic64_fetch_add() exists we can use it to return the base
context id, rather than the atomic64_add_return(N) - N concoction.

Suggested-by: Mika Kuoppala <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Reviewed-by: Maciej Patelczyk <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: ickle

drivers/dma-buf/dma-fence.c

@@ -106,7 +106,7 @@ EXPORT_SYMBOL(dma_fence_get_stub);
 u64 dma_fence_context_alloc(unsigned num)
 {
 	WARN_ON(!num);
-	return atomic64_add_return(num, &dma_fence_context_counter) - num;
+	return atomic64_fetch_add(num, &dma_fence_context_counter);
 }
 EXPORT_SYMBOL(dma_fence_context_alloc);