mctp: serial: cancel tx work on ldisc close

We want to ensure that the tx work has finished before returning from
the ldisc close op, so do a synchronous cancel.
Reported-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/mctp/mctp-serial.c

@@ -479,6 +479,7 @@ static void mctp_serial_close(struct tty_struct *tty)
 	int idx = dev->idx;
 
 	unregister_netdev(dev->netdev);
+	cancel_work_sync(&dev->tx_work);
 	ida_free(&mctp_serial_ida, idx);
 }