DAFS: Avoid unnecessary preattach on FSYNC_VOL_ON

DAFS: Avoid unnecessary preattach on FSYNC_VOL_ON

FSYNC_VOL_ON/FSYNC_VOL_ATTACH can be called to "online" a volume that
was actually kept online for the duration of the volume operation.
Avoid calling VPreAttachVolumeByVp_r for such a volume if it's already
attached, in order to avoid an unnecessary log message and to save a
tiny bit of processing.

Change-Id: I7776b933599ec0488941ccbd9a8e1279f5a552f4
Reviewed-on: http://gerrit.openafs.org/6167
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>