[php-src] PR #23574: ext/opcache: drop the pre-emptive munmap before the huge page remap

Pull Request: https://github.com/php/php-src/pull/23574 Author: s2x Follow-up to #23554, suggested by @arnaud-lb: MAP_FIXED already replaces the overlapped part of the reservation, so the munmap() before it is unnecessary and only opens a window for another thread to take the address. This keeps the reservation, maps the huge pages into it, and releases only the head and the tail that are left over — they always add up to huge_page_size. On failure the whole reservation is released, because a failed MAP_FIXED can leave a hole in it. As with my last patch, please check this carefully. This part of the code is still new to me.