Fix incorrect spare vdev state after replacing

After a hot spare replaces an OFFLINE vdev, the new
parent spare vdev state is set incorrectly to OFFLINE.
The correct state should be DEGRADED. The incorrect
OFFLINE state will prevent top-level vdev from reading
the spare vdev, thus causing unnecessary reconstruction.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Don Brady <don.brady@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Isaac Huang <he.huang@intel.com>
Closes #5766 
Closes #5770

module/zfs/spa.c

@@ -4713,6 +4713,11 @@ spa_vdev_attach(spa_t *spa, uint64_t guid, nvlist_t *nvroot, int replacing)
 	newvd->vdev_crtxg = oldvd->vdev_crtxg;
 	vdev_add_child(pvd, newvd);
 
+	/*
+	 * Reevaluate the parent vdev state.
+	 */
+	vdev_propagate_state(pvd);
+
 	tvd = newvd->vdev_top;
 	ASSERT(pvd->vdev_top == tvd);
 	ASSERT(tvd->vdev_parent == rvd);