volser: Close dirp on error in ConvertROtoRW

volser: Close dirp on error in ConvertROtoRW

Currently, if SAFSVolConvertROtoRWvolume cannot create a new transaction
for the volume to be converted, it returns without closing the directory
stream opened by it. To prevent this leak, go through a new 'goto done'
destructor if NewTrans fails.

Change-Id: Ie0580e7739ae667f1cd2f9cabb8aaf5e15d3f2dd
Reviewed-on: https://gerrit.openafs.org/14342
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>