Run a port scan and see for sure that port 21 is in fact being used first. Although this type of mixup is rare, you never know until you start checking the obvious.
Now, I really have no idea why xinetd would even be using this port. The error message:
Failed binding to 0.0.0.0, port 21: Address already in use
Did it mean to bind this to 0.0.0.0? hopefully not as, of course, it is not a real ip address