isobusfs: pack the protocol structures

The isobusfs protocol structures describe the message layout defined in
ISO 11783-13 but were not packed. The compiler added padding bytes in
front of 16/32 bit values which follow an odd number of bytes, e.g. the
Read File Request (8 bytes according to C.3.5.2) had a size of 10 bytes
and the count value at the wrong position. Server and client of the
can-utils used the same structures and understood each other but the
messages were not compatible to other ISOBUS devices. Additionally the
padding bytes sent uninitialized content.

Add __attribute__((__packed__)) to all protocol structures as it is
already done in j1939_vehicle_position.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>

isobusfs/isobusfs_cmn_va.h

@@ -29,7 +29,7 @@ struct isobusfs_va_init_vol_req {
 	uint8_t volume_flags;
 	__le16 name_len;
 	uint8_t name[];
-};
+} __attribute__((__packed__));
 
 /**
  * C.5.2.3 Initialize Volume Response
@@ -62,6 +62,6 @@ struct isobusfs_va_init_vol_res {
 	uint8_t tan;
 	uint8_t error_code;
 	uint8_t reserved[4];
-};
+} __attribute__((__packed__));
 
 #endif /* _ISOBUSFS_CMN_VA_H */