In lbd, atomic writes will require a pblk array in the lbd object.  Not sure
how to roll back partial allocations yet but it should be doable.

For async reads:
  - lbd_read() is called by compress_read() and compress_write().
    lbd may have multiple simultaneous callers.
    lbd calls lbatview_read() and reads its own data.
  - lbatview_read() is called by lbd.
    lbatview may have multiple simultaneous callers.
    lbatview calls pbat_read() and reads its own data.
  - pbat_read() is called by lbatview_alloc_pblk() and lbatview_free_pblk().
    pbat may have multiple simultaneous callers.
    pbat calls pbat_read().

Rework cache ownership:
  - compress_open() should alloc only lbdcache.
  - lbdcache should alloc only lbatviewcache.
  - lbatviewcache should alloc lbatpagecache and pbatcache.

Cache object sizing:
  - lbdcache size: multiple of num_online_cpus().
  - lbatviewcache:
    Min: one.
    Max: one per lbd.
    Avg: 1/2 lbdcache size.
    => Alloc 1/2 lbdcache size.
  - lbpatpage cache:
    Min: lbatviewcache size.
    Max: 2 * lbatviewcache size.
    Avg: 1.5 * lbatviewcache size.
    => alloc 1.5 * lbatviewcache size.
  - pbatcache size:
    Min: 1
    Max: lbatviewcache size.
    Avg: 1/2 lbatviewcache size.
    => alloc ???
       1/2 lbatviewcache size is way too large.
       Ratio of lbatview to pbat is 1:lbat_per_pbat.

Cache objects should dynamically expand.


TODO:
  - Move back to module based build system.
  - Make compression algorithm and speed/level selectable.
  - Create utilities for:
    - Resizing a compressed device.
    - Checking and repairing a compressed device.
  - Check parameters in compress_ctr:
    - Compressed device must be large enough.
    - Backing device must be large enough.
   - Remove workqueue.
  - (?) Function ptrs for reading and writing lblk_alloc.