dma-buf: fix an error pointer vs NULL bug

Smatch detected potential error pointer dereference.

    drivers/gpu/drm/drm_syncobj.c:888 drm_syncobj_transfer_to_timeline()
    error: 'fence' dereferencing possible ERR_PTR()

The error pointer comes from dma_fence_allocate_private_stub().  One
caller expected error pointers and one expected NULL pointers.  Change
it to return NULL and update the caller which expected error pointers,
drm_syncobj_assign_null_handle(), to check for NULL instead.

Fixes: f781f66 ("dma-buf: keep the signaling time of merged fences v3")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/b09f1996-3838-4fa2-9193-832b68262e43@moroto.mountain

Author: Dan Carpenter

drivers/dma-buf/dma-fence.c

@@ -160,7 +160,7 @@ struct dma_fence *dma_fence_allocate_private_stub(ktime_t timestamp)
 
 	fence = kzalloc(sizeof(*fence), GFP_KERNEL);
 	if (fence == NULL)
-		return ERR_PTR(-ENOMEM);
+		return NULL;
 
 	dma_fence_init(fence,
 		       &dma_fence_stub_ops,

drivers/gpu/drm/drm_syncobj.c

@@ -355,8 +355,8 @@ static int drm_syncobj_assign_null_handle(struct drm_syncobj *syncobj)
 {
 	struct dma_fence *fence = dma_fence_allocate_private_stub(ktime_get());
 
-	if (IS_ERR(fence))
-		return PTR_ERR(fence);
+	if (!fence)
+		return -ENOMEM;
 
 	drm_syncobj_replace_fence(syncobj, fence);
 	dma_fence_put(fence);