DeleteVolume should check ITSROVOL as a bit

Other bits may be asserted even if this is a RO vol.

Change-Id: Iff5256db25502b61b161ec068bd9d2a389f796c7
Reviewed-on: http://gerrit.openafs.org/11617
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>