drm/nouveau/vm: make each vma take a reference on its parent vm

Signed-off-by: Ben Skeggs <[e-mail]>

drivers/gpu/drm/nouveau/core/subdev/vm/base.c

···
 	}
 	mutex_unlock(&nv_subdev(vmm)->mutex);
 
-	vma->vm     = vm;
+	vma->vm = NULL;
+	nouveau_vm_ref(vm, &vma->vm, NULL);
 	vma->offset = (u64)vma->node->offset << 12;
 	vma->access = access;
 	return 0;
···
 	nouveau_vm_unmap_pgt(vm, vma->node->type != vmm->spg_shift, fpde, lpde);
 	nouveau_mm_free(&vm->mm, &vma->node);
 	mutex_unlock(&nv_subdev(vmm)->mutex);
+
+	nouveau_vm_ref(NULL, &vma->vm, NULL);
 }
 
 int