OpenZFS 7786 - zfs`vdev_online() needs better notification about state changes

OpenZFS 7786 - zfs`vdev_online() needs better notification about state changes

Authored by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Albert Lee <trisk@forkgnu.org>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: bunder2015 <omfgbunder@gmail.com>
OpenZFS-issue: https://www.illumos.org/issues/7786
OpenZFS-commit: http://github.com/openzfs/openzfs/commit/db8498f
Closes #6074