hammer2 - Add the vfs.hammer2.cluster_write sysctl back in

hammer2 - Add the vfs.hammer2.cluster_write sysctl back in

* Value may be 0 or 1, defaults to 0 (disabled).  Was previously
  enabled.

* By default H2 no longer sets B_CLUSTEROK on dirty buffers being
  written.  Theoretically this shouldn't be a problem because H2
  already uses a large 64KB buffer size.

* Should improve I/O throughput on high-performance SSDs by removing
  kernel overhead and should also work better with vfsync's dirty
  buffer scan which is already sorted.