nfsd: give up callbacks on revoked delegations

The delegation is no longer returnable, so I don't think there's much
point retrying the recall.

(I think it's worth asking why we even need separate CLOSED_DELEG and
REVOKED_DELEG states.  But treating them the same would currently cause
nfsd4_free_stateid to call list_del_init(&dp->dl_recall_lru) on a
delegation that the laundromat had unhashed but not revoked, incorrectly
removing it from the laundromat's reaplist or a client's dl_recall_lru.)

Signed-off-by: J. Bruce Fields <bfields@redhat.com>