CVE-2026-68119: tcp: initialize standalone TCP-AO response padding

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

tcp: initialize standalone TCP-AO response padding

tcpv4sendack() and tcpv6sendresponse() construct standalone TCP responses with TCP-AO options. The option length carries the actual MAC length, but the TCP header length includes the option rounded up to a four-byte boundary.

tcpaohashhdr() writes the MAC only. Thus, when the MAC length is not four-byte aligned, the one to three bytes after the MAC are left uninitialized and may be transmitted. For the normal TCP-AO hashing mode, those bytes also have to be initialized before computing the MAC.

Initialize only the alignment padding in the TCP-AO branches, before hashing the header. Use TCPOPTNOP, as in the normal TCP-AO output path. This avoids adding work to non-AO TCP responses while preserving a valid authenticated header.