canfd: add new data structures and constants

- add new struct canfd_frame
- check identical element offsets in struct can_frame and struct canfd_frame
- new ETH_P_CANFD definition to tag CAN FD skbs correctly
- add CAN_MTU and CANFD_MTU definitions for easy frame and mode detection
- add CAN[FD]_MAX_[DLC|DLEN] helper constants to remove hard coded values
- update existing struct can_frame with helper constants and comments

Signed-off-by: Oliver Hartkopp <[email protected]>
Signed-off-by: Marc Kleine-Budde <[email protected]>

net/can/af_can.c

@@ -41,6 +41,7 @@
  */
 
 #include <linux/module.h>
+#include <linux/stddef.h>
 #include <linux/init.h>
 #include <linux/kmod.h>
 #include <linux/slab.h>
@@ -824,6 +825,12 @@ static struct notifier_block can_netdev_notifier __read_mostly = {
 
 static __init int can_init(void)
 {
+	/* check for correct padding to be able to use the structs similarly */
+	BUILD_BUG_ON(offsetof(struct can_frame, can_dlc) !=
+		     offsetof(struct canfd_frame, len) ||
+		     offsetof(struct can_frame, data) !=
+		     offsetof(struct canfd_frame, data));
+
 	printk(banner);
 
 	memset(&can_rx_alldev_list, 0, sizeof(can_rx_alldev_list));