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 <[e-mail]>
Cc: VMware Graphics Reviewers <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: Zack Rusin <[e-mail]>
Reviewed-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Christian König <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vmwgfx/vmwgfx_surface.c

···
 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);