Unsynchronized global static shared across all WireGuard interfaces in wg_is_underload

Summary

wg_is_underload(:1577) static struct timespec last_underload shared across ALL wg interfaces and ALL CPUs. Cross-interface: attacker flooding interface A handshakes -> interface B demands cookies (DoS posture coupling). Non-atomic RMW on multi-word struct. Code admits Dont care about races.