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.

Reviewed-on: http://gerrit.openafs.org/10154
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 0cd5c63ebeef7567c9965e1ded7f10c6f452f0c3)

Change-Id: I51f4c08d1bcec49fc710d83e2c44e78653e607b7
Reviewed-on: http://gerrit.openafs.org/10164
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>