can: isotp: isotp_sendmsg(): fix return error fix on TX path

With commit d674a8f1 ("can: isotp: isotp_sendmsg(): fix return
error on FC timeout on TX path") the missing correct return value in
the case of a protocol error was introduced.

But the way the error value has been read and sent to the user space
does not follow the common scheme to clear the error after reading
which is provided by the sock_error() function. This leads to an error
report at the following write() attempt although everything should be
working.

Fixes: d674a8f1 ("can: isotp: isotp_sendmsg(): fix return error on FC timeout on TX path")
Reported-by: Carsten Schmidt <carsten.schmidt-achim@t-online.de>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/all/20230607072708.38809-1-socketcan@hartkopp.net
Cc: stable@vger.kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

net/can/isotp.c

@@ -1112,8 +1112,9 @@ static int isotp_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
 		if (err)
 			goto err_event_drop;
 
-		if (sk->sk_err)
-			return -sk->sk_err;
+		err = sock_error(sk);
+		if (err)
+			return err;
 	}
 
 	return size;