DAFS: Preattach, not attach, in FSYNC_Drop

FSYNC_Drop currently attaches volumes that were checked out by the
dropped fssync handler, but not checked back in, in order to make the
volume available again. For DAFS, however, a full attachment is
unnecessary; just preattach instead.

Reviewed-on: http://gerrit.openafs.org/7515
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit a43f0e4717fd72727ae325eb4437cef065419cdd)

Change-Id: Iaa2320808c4855516192b164de9778c9019a4559
Reviewed-on: http://gerrit.openafs.org/9481
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>