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.

Change-Id: I77af22caed9eceb4c90b8fa18b91945d059995e5
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>