9036 zfs: duplicate 'const' declaration specifier

illumos/illumos-gate@f02c28e434fb4d81d95122bab187fb43d4f19c2a

Reviewed by: Yuri Pankov <yuripv@yuripv.net>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Toomas Soome <tsoome@me.com>