DAFS: Do not attach a specialStatus'd vol

DAFS: Do not attach a specialStatus'd vol

If we encounter a preattached volume during GetVolume, we currently
ignore vp->specialStatus before trying to attach. However, we will
generally always fail to attach due to a conflicting vol op, but even
if we don't, GetVolume always returns an error later on if
vp->specialStatus is set. So, same some processing and attempted
attachments by bailing out sooner if vp->specialStatus is set.

Change-Id: Id5df08fe31073485e4d733c42310b42090ee8e7b
Reviewed-on: http://gerrit.openafs.org/4874
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>