OpenZFS 9036 - zfs: duplicate 'const' declaration specifier

Authored by: Toomas Soome <tsoome@me.com>
Reviewed by: Yuri Pankov <yuripv@yuripv.net>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://illumos.org/issues/9036
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/f02c28e434
Closes #6900