Well I didn't test that very well.  __page_cache_release() is doing a
__free_page() on a zero-ref page, so __free_pages() sends the refcount
negative and doesn't free it.  With patch #8, page_cache_release()
almost never frees pages, but it must have been leaking a little bit.
Lucky it showed up.

This fixes it, and also adds a missing PageReserved test in put_page().
Which makes put_page() identical to page_cache_release(), but there are
header file woes.  I'll fix that up later.