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.

Reviewed-on: http://gerrit.openafs.org/6167
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit d5d2d00a47cf53054bd18d7404be26bea34cba6f)

Change-Id: I2a7f4b214176570e787978dbe0aa2eb8dc57730f
Reviewed-on: http://gerrit.openafs.org/6323
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>