xdrgen: typedefs should use the built-in string and opaque functions

'typedef opaque yada<XYZ>' should use xdrgen's built-in opaque
encoder and decoder, to enable better compiler optimization.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

tools/net/sunrpc/xdrgen/templates/C/typedef/decoder/variable_length_opaque.j2

@@ -13,5 +13,5 @@ xdrgen_decode_{{ name }}(struct xdr_stream *xdr, {{ classifier }}{{ name }} *ptr
 {% if annotate %}
 	/* (variable-length opaque) */
 {% endif %}
-	return xdr_stream_decode_opaque(xdr, ptr->data, ptr->len) >= 0;
+	return xdrgen_decode_opaque(xdr, ptr, {{ maxsize }});
 };

tools/net/sunrpc/xdrgen/templates/C/typedef/decoder/variable_length_string.j2

@@ -13,5 +13,5 @@ xdrgen_decode_{{ name }}(struct xdr_stream *xdr, {{ classifier }}{{ name }} *ptr
 {% if annotate %}
 	/* (variable-length string) */
 {% endif %}
-	return xdr_stream_decode_opaque(xdr, ptr->data, ptr->len) >= 0;
+	return xdrgen_decode_string(xdr, ptr, {{ maxsize }});
 };