kernel - Fix degenerate cluster_write() cases

kernel - Fix degenerate cluster_write() cases

* cluster_write() should bdwrite() as a fallback, not bawrite().

  Note that cluster_awrite() always bawrite()'s or equivalent.  The
  DragonFly API split the functions out, so cluster_write() can now
  almost always bdwrite() for the non-clustered case.

* Solves some serious performance and real-time disk space usage issues
  when HAMMER1 was updated to use the cluster calls.  The disk space
  would be recovered by the daily cleanup but the extra writes could
  end up being quite excessive, 25:1 unnecessary writes vs necessary
  writes.

Reported-by: multiple, testing by tuxillo