volser: Indicate busy volume with VBUSY

Commit 34fc86bcc749f3bd059831b7e5dae03dc09a9393 changed several uses
of VBUSY to VOLSERVOLBUSY in order to detect retriable operations.
However, one such change did not change an Rx abort code, but instead
was used for the 'status' field for a volintInfo or volintXInfo
structure. That is not really a general error code, but a field with a
few specific known values (at least, that is how existing clients
interpret it).

Go back to using VBUSY, so clients indicate the volume as busy,
instead of as offline/unattached.

Reported by Andy Malato.

Reviewed-on: http://gerrit.openafs.org/9678
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 3929282d2767bd35e54037e5477e917979c84be3)

Change-Id: Ic6efeeca7f37d05704b09a3244bcf40641cae5ea
Reviewed-on: http://gerrit.openafs.org/9685
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>