CVE-2026-72398: sctp: add INIT verification after cookie unpacking

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

sctp: add INIT verification after cookie unpacking

In SCTP handshake, the INIT chunk is initially processed by the server and embedded into the cookie carried in INIT-ACK. The client then returns this cookie via COOKIE-ECHO, where the server unpacks it and reconstructs the original INIT chunk.

When cookie authentication is enabled, the cookie contents are protected against tampering, so reusing the unpacked INIT without re-verification is safe.

However, when cookie authentication is disabled, the reconstructed INIT can no longer be trusted. In this case, the INIT must be explicitly validated after unpacking to avoid processing potentially tampered data.

Add sctpverifyinit() checks after cookie unpacking in COOKIE-ECHO processing paths (sctpsfdo51Dce() and sctpsfdo524dupcook()) when cookieauthenable is disabled. On failure, the new association is freed and the packet is discarded.

Also tighten cookie validation in sctpunpackcookie() by verifying the embedded chunk type is SCTPCIDINIT before treating it as an INIT chunk.

Finally, update sctpverifyinit() to validate parameter bounds using the actual embedded INIT length instead of chunk->chunkend, since the INIT stored in COOKIE-ECHO may not span the entire chunk buffer.