drm/vmwgfx: Annotate struct vmw_surface_dirty 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 vmw_surface_dirty.

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

Cc: Zack Rusin <zackr@vmware.com>
Cc: VMware Graphics Reviewers <linux-graphics-maintainer@vmware.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Zack Rusin <zackr@vmware.com>
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/20230922173216.3823169-8-keescook@chromium.org

drivers/gpu/drm/vmwgfx/vmwgfx_surface.c

@@ -77,7 +77,7 @@ struct vmw_surface_offset {
 struct vmw_surface_dirty {
 	struct vmw_surface_cache cache;
 	u32 num_subres;
-	SVGA3dBox boxes[];
+	SVGA3dBox boxes[] __counted_by(num_subres);
 };
 
 static void vmw_user_surface_free(struct vmw_resource *res);