MFV 286597: 5701 zpool list reports incorrect "alloc" value for cache devices

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george@delphix.com>
Reviewed by: Alek Pinchuk <alek.pinchuk@nexenta.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Prakash Surya <prakash.surya@delphix.com>

illumos/illumos-gate@a52fc310ba80fa3b2006110936198de7f828cd94