TCP RACK: fix RSM computation for TCP fast open

[tuexen]

Summary

When spitting an RSM containing the SYN and data, honor the sequence number used by the SYN flag when computing the offset in the socket buffer.
This patch fixes PR 297006.

[maxim]

{F177033943}Hi Michael,

While your patch fixes 297006 panic, it still panics with Robert's code from 297273. From what I see the code path in 297273 is different.

I attached kyua tests for both cases generated with Claude Code.

Maxim

[tuexen]

In D61149#1401915, @maxim wrote:

{F177033943}Hi Michael,

While your patch fixes 297006 panic, it still panics with Robert's code from 297273. From what I see the code path in 297273 is different.

I agree. There was a problem on my side. I added packetdrill reproducers to the PRs.

I attached kyua tests for both cases generated with Claude Code.

Maxim