Windows: Delay MP Target evaluation until required

Do not evaluate the Target FID of a mount point until such time
as the target directory must be accessed.

Change-Id: I8aa0da50256cacc4433fad6c7675ca8bd3bd5eb7
Reviewed-on: http://gerrit.openafs.org/10154
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>