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.

Reviewed-on: http://gerrit.openafs.org/10828
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 939784c017f065a2f4fce19b48cdde56ac4be83c)

Change-Id: I59d2757db9a63a3569445549b2527b5885348508
Reviewed-on: http://gerrit.openafs.org/10918
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>