Pulling slabs out of struct page

> Those could be eliminated by splitting the structure into three allocator-specific variants, but that would add complication to a patch set that is already large (and set to grow).

I'm glad someone sees where I'm going! Yes, I did think about doing that, but decided not to do it as part of this patch. partial_pages will still need a union, but there will be far less unions once struct slab is defined based on CONFIG options.