CVE-2026-68376: sctp: fix auth_hmacs array size in struct sctp_cookie

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

sctp: fix authhmacs array size in struct sctpcookie

The authhmacs array in struct sctpcookie is supposed to store a complete SCTPAUTHHMACALGO parameter, which consists of a struct sctpparamhdr followed by N HMAC identifiers.

However, the array size was calculated using an extra 2 bytes instead of sizeof(struct sctpparamhdr), which is 4 bytes. When four HMAC identifiers are configured, the HMAC-ALGO parameter stored in the endpoint is larger than the authhmacs buffer in the cookie.

As a result, sctpassociationinit() copies beyond the end of authhmacs when initializing the association, corrupting the adjacent authchunks field. This can lead to an invalid HMAC identifier being accepted and later cause an out-of-bounds read in sctpauthgethmac().

Fix the array size calculation by including the full SCTP parameter header size.