CHROMIUM: dm-verity: initial cut

CHROMIUM: dm-verity: initial cut

First cut of dm-verity and dm-bht.  This implements a hash-based block
device verification dm module.

[It still needs work, but I'd love to be able to start doing iterative changes and not working on big, unchecked in blobs :)]

BUG=327
TESTS=automated:
      - dm-bht are coming in the verity cl
      - autotests need to be written for testing live
      manual:
      - by mounting /dev/sda5 with a rootfs on it and using a loop device with the hashes;
      - ran with slub debug FPZ: discovered dm-io bug which is no longer used in the code
      - ran dd on the raw device with block sizes from 4k - 1M and counts from 1 to full disk, skip as well
        - uncovered lock-ups in io_populate loop and oom conditions with initial dm_bht_populate flood of
          I/O requests (max_bios)
      - base tests on ref show ~25mb/s.  autotests to follow

Signed-off-by: Will Drewry <wad@chromium.org>
Review URL: http://codereview.chromium.org/1921007