avoid leaking stat info

avoid leaking stat info

if a file is already CStatd we just copy out the information without doing
access checks. add an access check.

Change-Id: I94d1dc118189b19759d8e9635988c40f38026cbb
Reviewed-on: http://gerrit.openafs.org/995
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>