kasan: test: Silence allocation warnings from GCC 12

kasan: test: Silence allocation warnings from GCC 12

GCC 12 is able to see more problems with allocation sizes at compile
time, so these must be silenced so the runtime checks will still be
available. Use OPTIMIZER_HIDE_VAR() to silence the new warning:

lib/test_kasan.c: In function 'ksize_uaf':
lib/test_kasan.c:781:61: warning: array subscript 120 is outside array bounds of 'void[120]' [-Warray-bounds]
  781 |         KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[size]);
      |                                       ~~~~~~~~~~~~~~~~~~~~~~^~~~~~
lib/test_kasan.c:96:9: note: in definition of macro 'KUNIT_EXPECT_KASAN_FAIL'
   96 |         expression;                                                     \
      |         ^~~~~~~~~~
In function 'kmalloc',
    inlined from 'ksize_uaf' at lib/test_kasan.c:775:8:
./include/linux/slab.h:581:24: note: at offset 120 into object of size 120 allocated by 'kmem_cache_alloc_trace'
  581 |                 return kmem_cache_alloc_trace(
      |                        ^~~~~~~~~~~~~~~~~~~~~~~
  582 |                                 kmalloc_caches[kmalloc_type(flags)][index],
      |                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  583 |                                 flags, size);
      |                                 ~~~~~~~~~~~~

Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: kasan-dev@googlegroups.com
Signed-off-by: Kees Cook <keescook@chromium.org>