CVE-2026-72021: ipvs: use parsed transport offset in SCTP state lookup

In the Linux kernel, the following vulnerability has been resolved:

ipvs: use parsed transport offset in SCTP state lookup

setsctpstate() reads the SCTP chunk header again in order to drive the IPVS SCTP state table. For IPv6 it computes the offset with sizeof(struct ipv6hdr), while the surrounding IPVS code uses iph.len from ipvsfilliphskb(), where ipv6findhdr() has already skipped extension headers and found the real transport header.

This makes the state machine read from the wrong offset for IPv6 SCTP packets that carry extension headers. For example, an INIT packet with an 8-byte destination options header can be scheduled correctly by sctpconnschedule(), but setsctpstate() reads the first byte of the SCTP verification tag as a DATA chunk type. The connection then moves from NONE to ESTABLISHED instead of INIT1, gets the longer established timeout, and updates the active/inactive destination counters incorrectly. This happens even though the SCTP handshake has not completed.

Use the parsed transport offset passed down from ipvssetstate() for the SCTP chunk-header lookup. For IPv4 and IPv6 packets without extension headers this preserves the existing offset.