drm/virtio: Annotate struct virtio_gpu_object_array with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct virtio_gpu_object_array.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: David Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Cc: Chia-I Wu <olvaffe@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230922173216.3823169-7-keescook@chromium.org

drivers/gpu/drm/virtio/virtgpu_drv.h

@@ -119,7 +119,7 @@ struct virtio_gpu_object_array {
 	struct ww_acquire_ctx ticket;
 	struct list_head next;
 	u32 nents, total;
-	struct drm_gem_object *objs[];
+	struct drm_gem_object *objs[] __counted_by(total);
 };
 
 struct virtio_gpu_vbuffer;