NFS: Dont clobber more uptodate values in nfs_set_verifier()

nfs_lookup_revalidate and friends are not serialised, so it is currently
quite possible for the dentry to be revalidated, and then have the
updated verifier replaced with an older value by another process.

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