NFS: Fix a typo in nfs_rename()

On successful rename, the "old_dentry" is retained and is attached to
the "new_dir", so we need to call nfs_set_verifier() accordingly.

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