MFV r318929: 7786 zfs`vdev_online() needs better notification about state changes

MFV r318929: 7786 zfs`vdev_online() needs better notification about state changes

illumos/illumos-gate@5f368aef86387d6ef4eda84030ae9b402313ee4c
https://github.com/illumos/illumos-gate/commit/5f368aef86387d6ef4eda84030ae9b402313ee4c

https://www.illumos.org/issues/7786
  Currently, vdev_online() will only post sysevent if previous state was
  "offline". It should also post the event when the state changes from "removed"
  or "faulted" to "healthy" or "degraded".
  This will fix the following scenario:
  - pull disk from slot A
  - check that hotspare has taken its place (if available)
  - insert disk into slot B
  - check that hotspare moved back to "avail" state (if spare was used)
  The problem here is that we don't get any ESC_ZFS_VDEV_* notification and fail
  to update the vdev FRU.

Reviewed by: Matthew Ahrens mahrens@delphix.com
Reviewed by: George Wilson george.wilson@delphix.com
Approved by: Albert Lee <trisk@forkgnu.org>
Author: Yuri Pankov <yuri.pankov@nexenta.com>

MFC after:	1 week