Windows: duplicate ExtentsResource hold

Windows: duplicate ExtentsResource hold

All callers to AFSDoExtentsMapRegion() hold the ExtentsResource
so there is no need to obtain it again within the function.  Instead
add an ASSERT to confirm that the resource is held.

Change-Id: Id34a3d650062d6e519a66732d248c136a92e4be3
Reviewed-on: http://gerrit.openafs.org/8161
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>