dm-pcache: add cache_key

Add *cache_key.c* which becomes the heart of dm-pcache’s
in-memory index and on-media key-set (“kset”) format.

* Key objects (`struct pcache_cache_key`)
  - Slab-backed allocator & ref-count helpers
  - `cache_key_encode()/decode()` translate between in-memory keys and
    their on-disk representation, validating CRC when
    *cache_data_crc* is enabled.

* Kset construction & persistence
  - Per-kset buffer lives in `struct pcache_cache_kset`; keys are
    appended until full or *force_close* triggers an immediate flush.
  - `cache_kset_close()` writes the kset to the *key_head* segment,
    automatically chaining a *LAST* kset header when rolling over to a
    freshly allocated segment.

* Red-black tree with striping
  - Cache space is divided into *subtrees* to reduce lock
    contention; each subtree owns its own RB-root + spinlock.
  - Complex overlap-resolution logic (`cache_insert_fixup()`) ensures
    newly inserted keys never leave overlapping stale ranges behind
    (head/tail/contain/contained cases handled).

* Replay on start-up
  - `cache_replay()` walks from *key_tail* to *key_head*, re-hydrates
    keys, validates CRC/magic, seamlessly
    skipping placeholder “empty” keys left by read-misses.

* Background maintenance
  - `clean_work` lazily prunes invalidated keys after GC.
  - `kset_flush_work` background thread to close a kset.

With this patch dm-pcache can persistently track cached extents, rebuild
its index after crash, and guarantee non-overlapping key space – paving
the way for functional read/write caching.

Signed-off-by: Dongsheng Yang <[email protected]>

Author: yangdongsheng