vol: Do not overwrite specialStatus in attach2

vol: Do not overwrite specialStatus in attach2

attach2 wants to set specialStatus to VBUSY in certain conditions
(such as, it discovers a conflicting vol op where VVolOpSetVBusy_r is
true). However, specialStatus may already be set to something else,
like VMOVED if the volume is being moved off of the server. This can
happen if the volserver has checked out and FSYNC_VOL_MOVE'd a
preattached volume but hasn't deleted or checked the volume back in
yet.

So, if specialStatus is already set, don't touch it, so we don't start
reporting VBUSY errors to clients when we should be reporting VMOVED,
or some other error code previously set.

Reviewed-on: http://gerrit.openafs.org/4873
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 25688bc2e7e8da83b4bf22d7cdc3e0214eadc455)

Change-Id: I4316be912a5a30856914059984f45782bece0cdd
Reviewed-on: http://gerrit.openafs.org/4942
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>