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 <[e-mail]>
Acked-by: Oliver Hartkopp <[e-mail]>
Signed-off-by: Marc Kleine-Budde <[e-mail]>

Documentation/networking/can.txt

···
     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

···
  * @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)));
 };