overflow: Change DEFINE_FLEX to take __counted_by member

overflow: Change DEFINE_FLEX to take __counted_by member

The norm should be flexible array structures with __counted_by
annotations, so DEFINE_FLEX() is updated to expect that. Rename
the non-annotated version to DEFINE_RAW_FLEX(), and update the
few existing users. Additionally add selftests for the macros.

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20240306235128.it.933-kees@kernel.org
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>