SUNRPC: Use xdr_stream to encode Reply verifier in svcauth_null_accept()

Done as part of hardening the server-side RPC header encoding path.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svcauth_unix.c

@@ -744,7 +744,6 @@ EXPORT_SYMBOL_GPL(svcauth_unix_set_client);
 static int
 svcauth_null_accept(struct svc_rqst *rqstp)
 {
-	struct kvec	*resv = &rqstp->rq_res.head[0];
 	struct xdr_stream *xdr = &rqstp->rq_arg_stream;
 	struct svc_cred	*cred = &rqstp->rq_cred;
 	u32 flavor, len;
@@ -773,12 +772,12 @@ svcauth_null_accept(struct svc_rqst *rqstp)
 	if (cred->cr_group_info == NULL)
 		return SVC_CLOSE; /* kmalloc failure - client must retry */
 
-	/* Put NULL verifier */
-	svc_putnl(resv, RPC_AUTH_NULL);
-	svc_putnl(resv, 0);
+	svcxdr_init_encode(rqstp);
+	if (xdr_stream_encode_opaque_auth(&rqstp->rq_res_stream,
+					  RPC_AUTH_NULL, NULL, 0) < 0)
+		return SVC_CLOSE;
 
 	rqstp->rq_cred.cr_flavor = RPC_AUTH_NULL;
-	svcxdr_init_encode(rqstp);
 	return SVC_OK;
 }