Harvested RDRAND output left unzeroed on the kernel stack

Summary

rdrand_rng_harvest collects RDRAND output into stack buffer randomness[RDRAND_SIZE+32] feeds to add_buffer_randomness_src but never clears buffer before returning. Raw HW RNG bytes persist on softclock callout stack until overwritten. Defense-in-depth/hygiene gap. Stand-alone impact negligible - requires separate kernel info-leak primitive to read stale softclock-stack bytes. CSPRNG pool further mixes material so raw RDRAND bytes not directly kernel RNG output. Reported as hardening opportunity consistent with kernel crypto hygiene expectations.