NFS: Fix up O_DIRECT results

if we read or wrote something, we must report it

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Reviewed-by: Jeff Layton <jlayton@poochiereds.net>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>