HAMMER de-dup in master

[Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>]

    Ok folks, Ilya's HAMMER de-dup is now in master, but before you get too
    excited please note the following:

    * The hammer filesystem must be upgraded to version 5 (from 4).  This
      doesn't actually change anything but 2.8 doesn't have the upgrade
      code yet so if you upgrade it in 2.9, 2.8 will refuse to mount the
      filesystem.

      This is on purpose for now.  In a week or two I'll MFC the upgrade
      to 2.8.

    * de-dup is highly experimental.  There is a known issue where reblocking
      can sometimes undo some of the de-dupping that was done.

    * de-dup is highly experimental, don't use it on a production server yet.

    * de-dupped data tends to be more fragmented (this is just a general
      thing that is true for de-dupping, no matter the filesystem).  This
      means you are trading off a little performance for the de-dupped
      blocks.

      It should only be used when disk space is a concern, and only if it
      would effective.  The 'hammer dedup-simulate' can be used to see
      how well de-dup might perform without actually doing it (this will run
      on 2.8 too).

    * Did I mention that de-dup is highly experimental?  It needs a lot
      more testing before I would consider it production-ready.

					-Matt
					Matthew Dillon 
					<dillon@backplane.com>