mm/sl*b: Differentiate struct slab fields by sl*b implementations

With a struct slab definition separate from struct page, we can go further and
define only fields that the chosen sl*b implementation uses. This means
everything between __page_flags and __page_refcount placeholders now depends on
the chosen CONFIG_SL*B. Some fields exist in all implementations (slab_list)
but can be part of a union in some, so it's simpler to repeat them than
complicate the definition with ifdefs even more.

We can also make it clear which fields in SLUB are only used with
CONFIG_SLUB_CPU_PARTIAL.

The patch doesn't change physical offsets of the fields, although it could be
done later - for example it's now clear that tighter packing in SLOB could be
possible.

This should also prevent accidental use of fields that don't exist in given
implementation. Before this patch virt_to_cache() and and cache_from_obj() was
visible for SLOB (albeit not used), although it relies on the slab_cache field
that isn't set by SLOB. With this patch it's now a compile error, so these
functions are now hidden behind #ifndef CONFIG_SLOB.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>