CVE-2024-36244: net/sched: taprio: extend minimum interval restriction to entire cycle too

In the Linux kernel, the following vulnerability has been resolved:

net/sched: taprio: extend minimum interval restriction to entire cycle too

It is possible for syzbot to side-step the restriction imposed by the blamed commit in the Fixes: tag, because the taprio UAPI permits a cycle-time different from (and potentially shorter than) the sum of entry intervals.

We need one more restriction, which is that the cycle time itself must be larger than N ETHZLEN bit times, where N is the number of schedule entries. This restriction needs to apply regardless of whether the cycle time came from the user or was the implicit, auto-calculated value, so we move the existing "cycle == 0" check outside the "if "(!new->cycletime)" branch. This way covers both conditions and scenarios.

Add a selftest which illustrates the issue triggered by syzbot.