mm/slab: simplify SLAB_* flag handling

SLUB is the only remaining allocator. We can therefore get rid of
the logic for allocator-specific flags:

* Merge SLAB_CACHE_FLAGS into SLAB_CORE_FLAGS.

* Remove CACHE_CREATE_MASK and instead mask out SLAB_DEBUG_FLAGS if
  !CONFIG_SLUB_DEBUG. SLAB_DEBUG_FLAGS is now defined
  unconditionally (no impact on existing code, which ignores it if
  !CONFIG_SLUB_DEBUG).

* Define SLAB_FLAGS_PERMITTED in terms of SLAB_CORE_FLAGS and
  SLAB_DEBUG_FLAGS (no functional change).

While at it also remove misleading comments that suggest that
multiple allocators are available.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>