DAFS: Preattach, not attach, in FSYNC_Drop

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.

Change-Id: I2c9d0b8cf0ddc7f4182347706b2c26d86f69725e
Reviewed-on: http://gerrit.openafs.org/7515
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>