drm/nouveau/instmem/gk20a: fix crash during error path

If a memory allocation fails when using the DMA allocator,
gk20a_instobj_dtor_dma() will be called on the failed instmem object.
At this time, node->handle might not be NULL despite the call to
dma_alloc_attrs() having failed. node->cpuaddr is the right member to
check for such a failure, so use it instead.
Reported-by: NVince Hsu <vinceh@nvidia.com>
Signed-off-by: NAlexandre Courbot <acourbot@nvidia.com>
Reviewed-by: NThierry Reding <treding@nvidia.com>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c

@@ -148,7 +148,7 @@ gk20a_instobj_dtor_dma(struct gk20a_instobj_priv *_node)
 	struct gk20a_instmem_priv *priv = (void *)nvkm_instmem(node);
 	struct device *dev = nv_device_base(nv_device(priv));
 
-	if (unlikely(!node->handle))
+	if (unlikely(!node->cpuaddr))
 		return;
 
 	dma_free_attrs(dev, _node->mem->size << PAGE_SHIFT, node->cpuaddr,