NFS: Fix nfs_verify_change_attribute()

We don't care about whether or not some other process on our client is
changing the directory while we're in nfs_lookup_revalidate(), because the
dcache will take care of ensuring local atomicity.
We can therefore remove the test for nfs_caches_unstable().

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>