dm-pcache: add cache_req

Introduce cache_req.c, the high-level engine that
drives I/O requests through dm-pcache. It decides whether data is served
from the cache or fetched from the backing device, allocates new cache
space on writes, and flushes dirty ksets when required.

* Read path
  - Traverses the striped RB-trees to locate cached extents.
  - Generates backing READ requests for gaps and inserts placeholder
    “empty” keys to avoid duplicate fetches.
  - Copies valid data directly from pmem into the caller’s bio; CRC and
    generation checks guard against stale segments.

* Write path
  - Allocates space in the current data segment via cache_data_alloc().
  - Copies data from the bio into pmem, then inserts or updates keys,
    splitting or trimming overlapped ranges as needed.
  - Adds each new key to the active kset; forces kset close when FUA is
    requested or the kset is full.

* Miss handling
  - create_cache_miss_req() builds a backing READ, optionally attaching
    an empty key.
  - miss_read_end_req() replaces the placeholder with real data once the
    READ completes, or deletes it on error.

* Flush support
  - cache_flush() iterates over all ksets and forces them to close,
    ensuring data durability when REQ_PREFLUSH is received.

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

Author: yangdongsheng