dma-buf/sync-file: fix logic error in new fence merge code

When the array is empty because everything is signaled we can't use
add_fence() to add something because that would filter the signaled
fence again.

Signed-off-by: Christian König <[e-mail]>
Fixes: 519f490db07e ("dma-buf/sync-file: fix warning about fence containers")
Reviewed-by: Thomas Hellström <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/dma-buf/sync_file.c

···
 	}
 
 	if (index == 0)
-		add_fence(fences, &index, dma_fence_get_stub());
+		fences[index++] = dma_fence_get_stub();
 
 	if (num_fences > index) {
 		struct dma_fence **tmp;