hammer2 - Add KVABIO support for hammer2

hammer2 - Add KVABIO support for hammer2

* Add KVABIO support for H2.  This allows H2 to manipulate the buffer
  cache without having to fully synchronize buffer data to all cpus,
  saving us a boatload of global IPIs.

* This more than doubles uncached read throughput from NVMe media.
  A simple test showed an increase from ~600 MBytes/sec to
  ~1400 MBytes/sec through the filesystem.  The IPI rate was reduced
  from 25000 IPIs/cpu/sec to less than 200 IPIs/cpu/sec.

  Read throughput was likely improved even further.  The NVMe device
  used for the test has roughly a ~1500 MB/sec cap.