vol: A GOING_OFFLINE volume should yield VOFFLINE

vol: A GOING_OFFLINE volume should yield VOFFLINE

Currently, GetVolume treats a volume in the VOL_STATE_GOING_OFFLINE
state the same as VOL_STATE_SHUTTING_DOWN, and so returns VNOVOL for a
GOING_OFFLINE volume, but these states are very different.

GOING_OFFLINE indicates that a volume should soon be in the UNATTACHED
state, so we should treat GOING_OFFLINE the same as UNATTACHED for
returning errors to the user. For UNATTACHED, we return specialStatus
if it's set, or VOFFLINE otherwise; so, just do the same for
GOING_OFFLINE.

Change-Id: Ia9500d2cfd90b8c15782a06025f39298810d0a99
Reviewed-on: http://gerrit.openafs.org/7290
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>