MFC r322241: MFV r322240: 8491 uberblock on-disk padding to reserve space for smoothl...

MFC r322241: MFV r322240: 8491 uberblock on-disk padding to reserve space for smoothly merging zpool checkpoint & MMP in ZFS

illumos/illumos-gate@79c2b812ee2010ebf20fdd92dc5f06b59000a94c
https://github.com/illumos/illumos-gate/commit/79c2b812ee2010ebf20fdd92dc5f06b59000a94c

https://www.illumos.org/issues/8491
  The zpool checkpoint feature in DxOS added a new field in the uberblock.
  The Multi-Modifier Protection Pull Request from ZoL adds two new fields in the
  uberblock (Reference: https://github.com/zfsonlinux/zfs/pull/6279).
  As these two changes come from two different sources and once upstreamed and
  deployed will introduce an incompatibility with each other we want
  to upstream a change that will reserve the padding for both of them so
  integration goes smoothly and everyone gets both features.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Olaf Faaland <faaland1@llnl.gov>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Serapheim Dimitropoulos <serapheim@delphix.com>

git-svn-id: svn://svn.freebsd.org/base/stable/10@323759 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f