drm/nouveau/vm: fix memory corruption when pgt allocation fails

If we return freed vm, nouveau_drm_open will happily call nouveau_cli_destroy,
which will try to free it again.

Reported-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

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

@@ -352,7 +352,7 @@ nouveau_vm_create(struct nouveau_vmmgr *vmm, u64 offset, u64 length,
 	u64 mm_length = (offset + length) - mm_offset;
 	int ret;
 
-	vm = *pvm = kzalloc(sizeof(*vm), GFP_KERNEL);
+	vm = kzalloc(sizeof(*vm), GFP_KERNEL);
 	if (!vm)
 		return -ENOMEM;
 
@@ -376,6 +376,8 @@ nouveau_vm_create(struct nouveau_vmmgr *vmm, u64 offset, u64 length,
 		return ret;
 	}
 
+	*pvm = vm;
+
 	return 0;
 }