kunit/fortify: Fix replaced failure path to unbreak __alloc_size

kunit/fortify: Fix replaced failure path to unbreak __alloc_size

The __alloc_size annotation for kmemdup() was getting disabled under
KUnit testing because the replaced fortify_panic macro implementation
was using "return NULL" as a way to survive the sanity checking. But
having the chance to return NULL invalidated __alloc_size, so kmemdup
was not passing the __builtin_dynamic_object_size() tests any more:

[23:26:18] [PASSED] fortify_test_alloc_size_kmalloc_const
[23:26:19]     # fortify_test_alloc_size_kmalloc_dynamic: EXPECTATION FAILED at lib/fortify_kunit.c:265
[23:26:19]     Expected __builtin_dynamic_object_size(p, 1) == expected, but
[23:26:19]         __builtin_dynamic_object_size(p, 1) == -1 (0xffffffffffffffff)
[23:26:19]         expected == 11 (0xb)
[23:26:19] __alloc_size() not working with __bdos on kmemdup("hello there", len, gfp)
[23:26:19] [FAILED] fortify_test_alloc_size_kmalloc_dynamic

Normal builds were not affected: __alloc_size continued to work there.

Use a zero-sized allocation instead, which allows __alloc_size to
behave.

Fixes: 4ce615e798a7 ("fortify: Provide KUnit counters for failure testing")
Fixes: fa4a3f86d498 ("fortify: Add KUnit tests for runtime overflows")
Link: https://lore.kernel.org/r/20240501232937.work.532-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>