Illumos #3349: zpool upgrade -V bumps the on disk version number

Illumos #3349: zpool upgrade -V bumps the on disk version number

3349 zpool upgrade -V bumps the on disk version number, but leaves
the in core version
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Christopher Siden <chris.siden@delphix.com>
Reviewed by: Matt Ahrens <matthew.ahrens@delphix.com>
Reviewed by: Richard Lowe <richlowe@richlowe.net>
Approved by: Dan McDonald <danmcd@nexenta.com>

References:
  illumos/illumos-gate@25345e466695fbe736faa53b8f3413d8e8f81981
  https://www.illumos.org/issues/3349

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>