IP header checksum left stale after kernel records RR/TS options on locally-delivered packets

Summary

ip_dooptions modifies header when recording RR(:1702-1704)/TS(:1745-1771) options but never recomputes ip_sum. Forwarded packets repaired by ip_output. Locally delivered RR/TS packets: stale checksum. Transport layers use pseudo-header no re-verify ip_sum. Long-standing 4.4BSD behavior, no integrity bypass.