This is a device mapper block compression driver.

Note:
  - Sectors are always 512 bytes (kernel constant).
  - Physical blocks are variable 512b .. 4kb.
  - Logical blocks are variable 4kb .. 4mb.
  - All integers are little endian.

Block device layout:
  - pblk[0] header (512 bytes)
    /* Offset 0: magic and version */
    - byte[4]               magic
    - u16                   version_major
    - u16                   version_minor
    /* Offset 8: parameters */
    - u16                   flags
    - u8                    compression
                              hi nybble: algorithm (1=lz4, 2=zlib, ...) [1]
                              lo nybble: level (1..9) [1]
    - u8                    pblk_shift (0..3) [3 = 4kb]
    - u8                    lblk_shift (1..10) [4 = 64kb (*)]
    - u8                    lba_elem_pblk_bytes (2, 4, 6)
    - u8                    pbat_shift [0]
    - u8                    pad
    - u32                   nr_zones
    - u32                   lblk_per_zone
    - byte[40]              reserved
    /* Offset 64: stats */
    - u64                   pblk_used
    - u64                   lblk_used
    - byte[48]              reserved
    /* Offset 128: reserved */
    - byte[384]             reserved
  - pblk[] zone ...         vector of zone

(*) The maximum lblk_shift may be (and usually is) less than 10 because
    lblk_alloc size must be <= pblk size.  When pblk_shift=1 (1024) and
    lblk_alloc_16_32, the maximum lblk_shift is 8 (256kb).

Device size limits:
  - Min: header plus one zone (header, pbat, lbat, data)
    Arbitrarily limit to 1mb (?)
  - Max: 2^48 pblk = 1eb

There are six different lblk_alloc sizes:
  Struct name           Size    lblk_size       device_pblk_size
  ===========           ====    =========       ================
  lblk_alloc_16_16      2+2*8   <= 64kb         <= 64k
  lblk_alloc_16_32      2+4*8   <= 64kb         <= 4g
  lblk_alloc_16_48      2+6*8   <= 64kb         <= 256t
  lblk_alloc_32_16      4+2*8   > 64kb          <= 64k
  lblk_alloc_32_32      4+4*8   > 64kb          <= 4g
  lblk_alloc_32_48      4+6*8   > 64kb          <= 256t


Zone layout:
  - byte[pblk_size*pbat_len]    Physical block allocation table (pbat)
  - lblk_alloc_x_y[]            Logical block allocation table (lbat)
                                Note: padded to pblk size
  - data[]                      Data
                                One pblk per bit in pbat