OpenZFS 8491 - uberblock on-disk padding to reserve space for smoothly merging zpool...

OpenZFS 8491 - uberblock on-disk padding to reserve space for smoothly merging zpool checkpoint & MMP in ZFS

The zpool checkpoint feature in DxOS added a new field in the uberblock.
The Multi-Modifier Protection Pull Request from ZoL adds three 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.

Porting Notes: Preserved MMP comments in uberblock struct.

Authored by: Serapheim Dimitropoulos <serapheim@delphix.com>
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>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/8491
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/d84fa5f
Closes #6390