sc_topology_change_time never initialized: premature TC timer expiry defeats topology change notification

Summary

sc_topology_change_time(if_bridgevar.h:358) NEVER assigned anywhere. M_ZERO zero-init in bridge_clone_create. bstp_timer_expired(&sc_topology_change_timer, 0)(:1465-1466) always returns true on first tick (uint16 value>=0 always true) -> immediately clears topology_change flags. TC active ~1s instead of expected max_age+forward_delay(~35s). Stale CAM entries persist on neighbors during topology transitions. Attacker who triggers topology changes exploits lingering forwarding entries. Fix: set sc_topology_change_time=max_age+forward_delay in bstp_initialization + when timers change.