NFS: fix nfs_verify_change_attribute

We always want to check that the verifier and directory
cache_change_attribute match. This also allows us to remove the 'wraparound
hack' for the cache_change_attribute. If we're only checking for equality,
then we don't care about wraparound issues.

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