mptcp: validate MPTcpExtInvalidOptionRx on MP_CAPABLE fallback paths

[Dwyane-Yan]

The mptcp_net-next kernel now increments the MPTcpExtInvalidOptionRx MIB counter whenever the MPTCP option parser ignores an option due to a wrong version, disallowed flags, bad length, etc.

These four packetdrill tests already inject malformed MP_CAPABLE options that are rejected by the parser, causing a TCP fallback:

• v1_bind_tcpfallback_wrongver: v0 SYN (version < v1)
• v1_connect_tcpfallback_wrongver: v0 SYN-ACK (version != v1)
• v1_bind_tcpfallback_flagB: extensibility flag set
• v1_bind_tcpfallback_flagH: HMAC-SHA256 flag missing

Extend their nstat validation to also check that MPTcpExtInvalidOptionRx has been incremented. The connect-side wrong-version test had no nstat section, so add the nstat -n initialization along with the new assertion.

Assisted-by: Claude: GLM-5.2

[matttbe]

Thank you for this PR. Indeed, all of them should not have the MIB counter increased. Maybe we could keep this PR but check that the counter is 0 as expected?

We could also have another test sending invalid options, e.g. 2 different DSS (if packetdrill allows that in the current implementation) or an MPC + a DSS, etc.

Note that on my side, I called the counter OptInvalid → Opt for the prefix just in case other Opt related are added, and no Rx because I don't see why we would have invalid options on the output side :)

[Dwyane-Yan]

Sounds good, I can do this after your patches are sent to the mail list.

Thanks

[matttbe]

Thanks!

Reopening and converting to a draft, just not to forget about it.