OpenZFS 7386 - zfs get does not work properly with bookmarks

Authored by: Marcel Telka <marcel@telka.sk>
Reviewed by: Simon Klinkert <simon.klinkert@gmail.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7386
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/edb901a
Closes #5666