OpenZFS 4521 - zfstest is trying to execute evil "zfs unmount -a"

OpenZFS 4521 - zfstest is trying to execute evil "zfs unmount -a"

Authored by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Porting Notes:
- Correctly set __ZFS_POOL_RESTRICT in inherit_001_pos

OpenZFS-issue: https://www.illumos.org/issues/4521
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/8808ac5
Closes #5674