changing space map blksz, commit from illumos

[Matthew Ahrens]

I would like to change the space map block size on FreeBSD, to improve
performance on a system with very large vdevs.  This was made possible in
illumos (exactly!) a year ago by the commit below.  Does anyone know if
there's a technical reason that this was not ported to FreeBSD?  If not, it
would be great if someone wants to pick it up, otherwise I'll try to do so
when I have time.

commit b1be2892dd07cf9a97d47ad06334cdc879196aaf
Author: Matthew Ahrens <mahrens at delphix.com>
Date:   Tue Sep 16 11:53:58 2014 -0800

    5164 space_map_max_blksz causes panic, does not work
    5165 zdb fails assertion when run on pool with recently-enabled
spacemap_histogram feature
    Reviewed by: Christopher Siden <christopher.siden at delphix.com>
    Reviewed by: George Wilson <george.wilson at delphix.com>
    Reviewed by: Saso Kiselkov <skiselkov.ml at gmail.com>
    Approved by: Dan McDonald <danmcd at omniti.com>

--matt