mm/slub: refactor freelist to use custom type

Currently the SLUB code represents encoded freelist entries as "void*".
That's misleading, those things are encoded under
CONFIG_SLAB_FREELIST_HARDENED so that they're not actually dereferencable.

Give them their own type, and split freelist_ptr() into one function per
direction (one for encoding, one for decoding).

Signed-off-by: Jann Horn <jannh@google.com>
Co-developed-by: Matteo Rizzo <matteorizzo@google.com>
Signed-off-by: Matteo Rizzo <matteorizzo@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>