The odd thing about hugetlb is that it maintains its own freelist of pages.
And it has to do that, else it would trivially run out of pages due to buddy
fragmetation.

So we we don't want callers of put_page() to be passing those pages
to __free_pages_ok() on the final put().

So hugetlb installs a destructor in the compound pages to point at
free_huge_page(), which knows how to put these pages back onto the free list.

Also, don't mark hugepages as all PageReserved any more.  That's preenting
callers from doing proper refcounting.  Any code which does a user pagetable
walk and hits part of a hugepage will now handle it transparently.