kasan: clean up metadata allocation and usage

KASAN marks caches that are sanitized with the SLAB_KASAN cache flag.
Currently if the metadata that is appended after the object (stores e.g.
stack trace ids) doesn't fit into KMALLOC_MAX_SIZE (can only happen with
SLAB, see the comment in the patch), KASAN turns off sanitization
completely.

With this change sanitization of the object data is always enabled.
However the metadata is only stored when it fits. Instead of checking for
SLAB_KASAN flag accross the code to find out whether the metadata is
there, use cache->kasan_info.alloc/free_meta_offset. As 0 can be a valid
value for free_meta_offset, introduce KASAN_NO_FREE_META as an indicator
that the free metadata is missing.

Along the way rework __kasan_cache_create() and add claryfying comments.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Link: https://linux-review.googlesource.com/id/Icd947e2bea054cb5cfbdc6cf6652227d97032dcb