9084 spa_*_ashift must ignore spare devices
illumos/illumos-gate@b037f3dbd69cef4a7ffd576ad33e07bfaf0b1e84
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Prakash Surya <prakash.surya@delphix.com>