mm: Split slab into its own type

mm: Split slab into its own type

Make struct slab independent of struct page.  It still uses the
underlying memory in struct page for storing slab-specific data,
but slab and slub can now be weaned off using struct page directly.
Some of the wrapper functions (slab_address() and slab_order())
still need to cast to struct page, but this is a significant
disentanglement.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>