Windows: Fix RDR_BkgFetch rock assigment

The wrong variables were assigned to the rock base and offset
in the final queuing of a RDR_BkgFetch request within
RDR_RequestFileExtentsAsync.

Introduced in 8b874aa205cd05b8109a296c8cb8ad0bb5114553.

Reviewed-on: http://gerrit.openafs.org/9058
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 5f17b9f9295b4f1c86cc71a5253f93a9c56078a4)

Change-Id: Ib7229763a7dd35c58450c69da87d168057248a68
Reviewed-on: http://gerrit.openafs.org/9060
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>