Divide-by-zero kernel panic in mesh_airtime_calc via ni_txrate==0

Summary

mesh_airtime_calc(:3383) nbits/rate where rate=ni->ni_txrate. If ni_txrate=0 (freshly discovered neighbor no rate set) -> div-by-zero panic. Reachable from mesh_recv_action_meshlmetric(:2560) with no established-peer requirement. Remote single action frame -> kernel panic.