7233 dir_is_empty should open directory with CLOEXEC

illumos/illumos-gate@d420209d9c807f782c1d31f5683be74798142198
https://github.com/illumos/illumos-gate/commit/d420209d9c807f782c1d31f5683be74798142198

https://www.illumos.org/issues/7233

This fixes a race where one thread is executing zfs_mount() while another
thread forks and execs. If the fork occurs while the directory is open, the
child process will inherit (but not necessarily close immediately) the open fd
for the directory, preventing the mount.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Alex Reece <alex@delphix.com>