SET_TICKS computes len*8*dn_hz as signed int: overflow for jumbo at high dn_hz — same as dummynet v1 unfixed

Summary

SET_TICKS(:450-452) and ready_event(:502)/ready_event_wfq(:568): len*8*dn_hz evaluated as signed int32. len=65535,dn_hz=10000 -> 5.24e9 > INT_MAX -> wraps negative. Negative len_scaled corrupts credit accounting -> shaper bypass/starvation. Same unfixed bug as DF-0378 (v1). Fix: use (uint64_t)len*8*dn_hz.