Windows: AFSShareWrite do not assign pFcb too soon

In AFSShareWrite the value of pFcb is used to determine whether
or not the pfcb->NPFcb->Resource must be released upon exit.
Therefore, it must not be assigned a value until just before the
resource is acquired.

Change-Id: I6951d91fcf672c8d236d19e075fff0be3552c9b8
Reviewed-on: http://gerrit.openafs.org/10828
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>