lkdtm/heap: Avoid __alloc_size hint warning for VMALLOC_LINEAR_OVERFLOW

lkdtm/heap: Avoid __alloc_size hint warning for VMALLOC_LINEAR_OVERFLOW

Once __alloc_size hints have been added, the compiler will (correctly!)
see this as an overflow. We are, however, trying to test for this
condition at run-time (not compile-time), so work around it with a
volatile int offset.

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210818174855.2307828-5-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>