lib/test_kasan_module.c: suppress unused var warning

Local `unused' is intentionally unused - it is there to suppress
__must_check warnings.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lkml.kernel.org/r/202104050216.HflRxfJm-lkp@intel.com
Cc: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>