fortify: Adjust KUnit test for modular build

A much better "unknown size" string pointer is available directly from
struct test, so use that instead of a global that isn't shared with
modules.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/lkml/YyCOHOchVuE/E7vS@dev-arch.thelio-3990X


Fixes: 875bfd52 ("fortify: Add KUnit test for FORTIFY_SOURCE internals")
Cc: linux-hardening@vger.kernel.org
Build-tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>