can: j1939: recvmsg(): allow MSG_CMSG_COMPAT flag

The control message provided by J1939 support MSG_CMSG_COMPAT but
blocked recvmsg() syscalls that have set this flag, i.e. on 32bit user
space on 64 bit kernels.

Link: https://github.com/hartkopp/can-isotp/issues/59
Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Suggested-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Tested-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Fixes: 9d71dd0c ("can: add support of SAE J1939 protocol")
Link: https://lore.kernel.org/20230505110308.81087-3-mkl@pengutronix.de
Cc: stable@vger.kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

net/can/j1939/socket.c

@@ -798,7 +798,7 @@ static int j1939_sk_recvmsg(struct socket *sock, struct msghdr *msg,
 	struct j1939_sk_buff_cb *skcb;
 	int ret = 0;
 
-	if (flags & ~(MSG_DONTWAIT | MSG_ERRQUEUE))
+	if (flags & ~(MSG_DONTWAIT | MSG_ERRQUEUE | MSG_CMSG_COMPAT))
 		return -EINVAL;
 
 	if (flags & MSG_ERRQUEUE)