nfsd4: allow an EXCHANGE_ID to kill a 4.0 client

Following rfc 5661 section 2.4.1, we can permit a 4.1 client to remove
an established 4.0 client's state.

(But we don't allow updates.)
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -1553,8 +1553,10 @@ nfsd4_exchange_id(struct svc_rqst *rqstp,
 	conf = find_confirmed_client_by_str(dname, strhashval);
 	if (conf) {
 		if (!clp_used_exchangeid(conf)) {
-			status = nfserr_clid_inuse; /* XXX: ? */
-			goto out;
+			if (exid->flags & EXCHGID4_FLAG_UPD_CONFIRMED_REC_A) {
+				status = nfserr_inval; /* buggy client */
+				goto out;
+			}
 		}
 		if (!same_creds(&conf->cl_cred, &rqstp->rq_cred)) {
 			/* 18.35.4 case 9 */
@@ -1574,6 +1576,10 @@ nfsd4_exchange_id(struct svc_rqst *rqstp,
 			expire_client(conf);
 			goto out_new;
 		}
+		if (!clp_used_exchangeid(conf)) {
+			status = nfserr_inval;
+			goto out;
+		}
 		/*
 		 * Set bit when the owner id and verifier map to an already
 		 * confirmed client id (18.35.3).