greybus: loopback: add response len to loopback protocol

This patch adds a len field to the loopback protocol.
This field is validated in gb_loopback_transfer() and stuffed in
gb_loopback_request_recv().
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/greybus_protocols.h

@@ -917,6 +917,7 @@ struct gb_loopback_transfer_request {
 } __packed;
 
 struct gb_loopback_transfer_response {
+	__le32	len;
 	__u8	data[0];
 } __packed;
 

drivers/staging/greybus/loopback.c

@@ -524,6 +524,7 @@ static int gb_loopback_request_recv(u8 type, struct gb_operation *operation)
 				return -ENOMEM;
 			}
 			response = operation->response->payload;
+			response->len = cpu_to_le32(len);
 			memcpy(response->data, request->data, len);
 		}
 		return 0;