Add illumos FMD ZFS logic to ZED -- phase 2

The phase 2 work primarily entails the Diagnosis Engine and
the Retire Agent modules. It also includes infrastructure
to support a crude FMD environment to host these modules.

The Diagnosis Engine consumes I/O and checksum ereports and
feeds them into a SERD engine which will generate a corres-
ponding fault diagnosis when the SERD engine fires. All the
diagnosis state data is collected into cases, one case per
vdev being tracked.

The Retire Agent responds to diagnosed faults by isolating
the faulty VDEV. It will notify the ZFS kernel module of
the new VDEV state (degraded or faulted). This agent is
also responsible for managing hot spares across pools.
When it encounters a device fault or a device removal it
replaces the device with an appropriate spare if available.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@intel.com>
Closes #5343

module/zfs/vdev.c

@@ -3373,6 +3373,17 @@ vdev_set_state(vdev_t *vd, boolean_t isopen, vdev_state_t state, vdev_aux_t aux)
 	spa_t *spa = vd->vdev_spa;
 
 	if (state == vd->vdev_state) {
+		/*
+		 * Since vdev_offline() code path is already in an offline
+		 * state we can miss a statechange event to OFFLINE. Check
+		 * the previous state to catch this condition.
+		 */
+		if (vd->vdev_ops->vdev_op_leaf &&
+		    (state == VDEV_STATE_OFFLINE) &&
+		    (vd->vdev_prevstate >= VDEV_STATE_FAULTED)) {
+			/* post an offline state change */
+			zfs_post_state_change(spa, vd, vd->vdev_prevstate);
+		}
 		vd->vdev_stat.vs_aux = aux;
 		return;
 	}