kunit/overflow: Adjust for __counted_by with DEFINE_RAW_FLEX()

When a flexible array structure has a __counted_by annotation, its use
with DEFINE_RAW_FLEX() will result in the count being zero-initialized.
This is expected since one doesn't want to use RAW with a counted_by
struct. Adjust the tests to check for the condition and for compiler
support.

Reported-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Closes: https://lore.kernel.org/all/0bfc6b38-8bc5-4971-b6fb-dc642a73fbfe@gmail.com/


Suggested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20240610182301.work.272-kees@kernel.org


Tested-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Signed-off-by: Kees Cook <kees@kernel.org>