NFS: Don't force a revalidation of all attributes if change is missing

Even if the change attribute is missing, it is still OK to mark the other
attributes as being up to date.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>