Latest commit a0aacd3 Jan 18, 2017 @gmelikov gmelikov committed with behlendorf OpenZFS 7257 - zfs manpage user property length needs to be updated
Since zpool version 16, this limit is actually 8192 characters.
Additionally, this limit is actually 8192 bytes, as it supports UTF-8.

Authored by: Eli Rosenthal <eli.rosenthal@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7257
OpenZFS-commit: openzfs/openzfs@3bc7169
Closes #5608