OpenZFS 6544 - incorrect comment in libzfs.h about offline status

6544 incorrect comment in libzfs.h about offline status
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Ported-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/6544
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/cb605c4
Closes #4595