vol: Pay attention to specialStatus after VAVByVp

vol: Pay attention to specialStatus after VAVByVp

attach2/VAttachVolumeByVp_r do not alter the yielded error code
according to specialStatus. So, pay attention to specialStatus after
receiving an error from VAttachVolumeByVp_r, to ensure we respond with
the correct error code.

Change-Id: I59e977dd1f0949f8fe5670c7a52429acbfb7d7e9
Reviewed-on: http://gerrit.openafs.org/7303
Reviewed-by: Tom Keiser <tkeiser@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>