[$] Pulling slabs out of struct page

For the time being, the effort to add the folioconcept to the memory-management subsystem appears to be stalled, but appearances canbe deceiving. The numerous folio discussions have produced a number ofpoints of consensus, though; one of those is that far too much of thekernel has to work with page structures to get its job done. Asan example of how a subsystem might be weaned off of struct pageusage, Matthew Wilcox has split outthe slab allocators in a 62-part patch set. The result may bea foreshadowing of changes to come in the memory-management subsystem.