NFSv4: handle no acl attr

 Stop handing garbage to userspace in the case where a weird server clears the
 acl bit in the getattr return (despite the fact that they've already claimed
 acl support.)

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>