NFSv4: Ensure we don't re-test revoked and freed stateids

This fixes a potential infinite loop in nfs_reap_expired_delegations.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Tested-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>