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 * ETH_ZLEN 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->cycle_time)"

branch. This way covers both conditions and scenarios.

Add a selftest which illustrates the issue triggered by syzbot.