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