Windows: Flush data and then drop locks

Windows: Flush data and then drop locks

The redirector must flush the data to the file server
and then drop the file locks.  Doing it the other way
creates races.

Change-Id: Ic3a48546ee9e0066df9d228fc4e57f52b43587c2
Reviewed-on: http://gerrit.openafs.org/7207
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>