OpenZFS 7233 - dir_is_empty should open directory with CLOEXEC

Authored by: Alex Reece <alex@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.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/7233
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/d420209
Closes #5623