OpenZFS 7072 - zfs fails to expand if lun added when os is in shutdown state

OpenZFS 7072 - zfs fails to expand if lun added when os is in shutdown state

Authored by: George Wilson <george.wilson@delphix.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Alex Reece <alex@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
OpenZFS-issue: https://www.illumos.org/issues/7072
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/c39a2aa
Closes #5694

Porting notes:
- vdev.c: 'vdev_get_stats' changes are moved to 'vdev_get_stats_ex'.
- vdev_disk.c: ignored, Linux specific code is different.