dm-pcache: add cache core

Add cache.c and cache.h that introduce the top-level
“struct pcache_cache”. This object glues together the backing block
device, the persistent-memory cache device, segment array, RB-tree
indexes, and the background workers for write-back and garbage
collection.

* Persistent metadata
  - pcache_cache_info tracks options such as cache mode, data-crc flag
    and GC threshold, written atomically with CRC+sequence.
  - key_tail and dirty_tail positions are double-buffered and recovered
    at mount time.

* Segment management
  - kvcalloc()’d array of pcache_cache_segment objects, bitmap for fast
    allocation, refcounts and generation numbers so GC can invalidate
    old extents safely.
  - First segment hosts a pcache_cache_ctrl block shared by all
    threads.

* Request path hooks
  - pcache_cache_handle_req() dispatches READ, WRITE and FLUSH bios to
    the engines added in earlier patches.
  - Per-CPU data_heads support lock-free allocation of space for new
    writes.

* Background workers
  - Delayed work items for write-back (5 s) and GC (5 s).
  - clean_work removes stale keys after segments are reclaimed.

* Lifecycle helpers
  - pcache_cache_start()/stop() bring the cache online, replay keys,
    start workers, and flush everything on shutdown.

With this piece in place dm-pcache has a fully initialised cache object
capable of serving I/O and maintaining its on-disk structures.

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

Author: yangdongsheng