MFV r296538: 6544 incorrect comment in libzfs.h about offline status

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Gerhard Roethlin <git@the-color-black.net>

illumos/illumos-gate@cb605c4d8ab24b5a900b8b4ca85db65c22d05fad