nfsd4: return serverfault on request for ssv

We're refusing to support a mandatory features of 4.1, so serverfault
seems the better error; see e.g.:

	http://www.ietf.org/mail-archive/web/nfsv4/current/msg07638.htmlSigned-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -1344,7 +1344,7 @@ nfsd4_exchange_id(struct svc_rqst *rqstp,
 	case SP4_NONE:
 		break;
 	case SP4_SSV:
-		return nfserr_encr_alg_unsupp;
+		return nfserr_serverfault;
 	default:
 		BUG();				/* checked by xdr code */
 	case SP4_MACH_CRED: