drm/gud: Use size_add() in call to struct_size()

If, for any reason, the open-coded arithmetic causes a wraparound, the
protection that `struct_size()` adds against potential integer overflows
is defeated. Fix this by hardening call to `struct_size()` with `size_add()`.

Fixes: 40e1a70 ("drm: Add GUD USB Display driver")
Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/ZQSlyHKPdw/zsy4c@work
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: GustavoARSilva

drivers/gpu/drm/gud/gud_pipe.c

@@ -503,7 +503,7 @@ int gud_pipe_check(struct drm_simple_display_pipe *pipe,
 		return -ENOENT;
 
 	len = struct_size(req, properties,
-			  GUD_PROPERTIES_MAX_NUM + GUD_CONNECTOR_PROPERTIES_MAX_NUM);
+			  size_add(GUD_PROPERTIES_MAX_NUM, GUD_CONNECTOR_PROPERTIES_MAX_NUM));
 	req = kzalloc(len, GFP_KERNEL);
 	if (!req)
 		return -ENOMEM;