can.h: make padding given by gcc explicit

The current definition of struct can_frame has a 16-byte size, with 8-byte
alignment, but the 3 bytes of padding are not explicit like the similar 2 bytes
of padding of struct canfd_frame. Make it explicit so it is easier to read.

Signed-off-by: Shawn Landden <shawn@churchofgit.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Documentation/networking/can.txt

@@ -268,6 +268,9 @@ solution for a couple of reasons:
     struct can_frame {
             canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
             __u8    can_dlc; /* frame payload length in byte (0 .. 8) */
+            __u8    __pad;   /* padding */
+            __u8    __res0;  /* reserved / padding */
+            __u8    __res1;  /* reserved / padding */
             __u8    data[8] __attribute__((aligned(8)));
     };
 

include/uapi/linux/can.h

@@ -95,11 +95,17 @@ typedef __u32 can_err_mask_t;
  * @can_dlc: frame payload length in byte (0 .. 8) aka data length code
  *           N.B. the DLC field from ISO 11898-1 Chapter 8.4.2.3 has a 1:1
  *           mapping of the 'data length code' to the real payload length
+ * @__pad:   padding
+ * @__res0:  reserved / padding
+ * @__res1:  reserved / padding
  * @data:    CAN frame payload (up to 8 byte)
  */
 struct can_frame {
 	canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
 	__u8    can_dlc; /* frame payload length in byte (0 .. CAN_MAX_DLEN) */
+	__u8    __pad;   /* padding */
+	__u8    __res0;  /* reserved / padding */
+	__u8    __res1;  /* reserved / padding */
 	__u8    data[CAN_MAX_DLEN] __attribute__((aligned(8)));
 };