lkdtm/heap: Hide allocation size from -Warray-bounds

With the kmalloc() size annotations, GCC is smart enough to realize that
LKDTM is intentionally writing past the end of the buffer. This is on
purpose, of course, so hide the buffer from the optimizer. Silences:

../drivers/misc/lkdtm/heap.c: In function 'lkdtm_SLAB_LINEAR_OVERFLOW':
../drivers/misc/lkdtm/heap.c:59:13: warning: array subscript 256 is outside array bounds of 'void[1020]' [-Warray-bounds]
   59 |         data[1024 / sizeof(u32)] = 0x12345678;
      |         ~~~~^~~~~~~~~~~~~~~~~~~~
In file included from ../drivers/misc/lkdtm/heap.c:7:
In function 'kmalloc',
    inlined from 'lkdtm_SLAB_LINEAR_OVERFLOW' at ../drivers/misc/lkdtm/heap.c:54:14:
../include/linux/slab.h:581:24: note: at offset 1024 into object of size 1020 allocated by 'kmem_cache_alloc_trace'
  581 |                 return kmem_cache_alloc_trace(
      |                        ^~~~~~~~~~~~~~~~~~~~~~~
  582 |                                 kmalloc_caches[kmalloc_type(flags)][index],
      |                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  583 |                                 flags, size);
      |                                 ~~~~~~~~~~~~

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kees Cook <keescook@chromium.org>

drivers/misc/lkdtm/heap.c

@@ -56,6 +56,7 @@ static void lkdtm_SLAB_LINEAR_OVERFLOW(void)
 		return;
 
 	pr_info("Attempting slab linear overflow ...\n");
+	OPTIMIZER_HIDE_VAR(data);
 	data[1024 / sizeof(u32)] = 0x12345678;
 	kfree(data);
 }