nfsd: adjust WARN_ON_ONCE in revoke_delegation

A WARN_ON_ONCE() is added to revoke delegations to make sure that the
state has been marked for revocation. However, that's only true for 4.1+
stateids. For 4.0 stateids, in unhash_delegation_locked() the sc_status
is set to SC_STATUS_CLOSED. Modify the check to reflect it, otherwise
a WARN_ON_ONCE is erronously triggered.

Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>