zfsd: don't try to fix an OFFLINE condition

If the system administrator does "zpool offline", he's doing it for a
reason. zfsd shouldn't consider an offline disk to be an event that
requires automatic healing. Don't online it in response to a GEOM
event, and don't try to activate a hotspare to take over from it.

Sponsored by: ConnectWise

(cherry picked from commit a13ddd6210f349dec341eba0506b5f2395a7a2d6)