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 <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Maciej Patelczyk <maciej.patelczyk@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200513074809.18194-2-chris@chris-wilson.co.uk

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);