CLEANUP: pools: move the write before free to the uaf-only function

CLEANUP: pools: move the write before free to the uaf-only function

In UAF mode, pool_put_to_os() performs a write to the about-to-be-freed
memory area so as to make sure the page is properly mapped and catch a
possible double-free. However there's no point keeping that in an ifdef
in the generic function, because we now have a pool_free_area_uaf()
that is the UAF-specific version of pool_free_area() and the one that
is called immediately after this write. Let's move the code there, it
will be cleaner.