[$] Pulling slabs out of struct page

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