randstruct: Fix gcc-plugin performance mode to stay in group

The performance mode of the gcc-plugin randstruct was shuffling struct
members outside of the cache-line groups. Limit the range to the
specified group indexes.

Cc: linux-hardening@vger.kernel.org
Cc: stable@vger.kernel.org
Reported-by: Lukas Loidolt <e1634039@student.tuwien.ac.at>
Closes: https://lore.kernel.org/all/f3ca77f0-e414-4065-83a5-ae4c4d25545d@student.tuwien.ac.at


Fixes: 313dd1b6 ("gcc-plugins: Add the randstruct plugin")
Signed-off-by: Kees Cook <keescook@chromium.org>