CVE-2022-49325: Reachable Assertion

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

tcp: add accessors to read/set tp->snd_cwnd

We had various bugs over the years with code breaking the assumption that tp->snd_cwnd is greater than zero.

Lately, syzbot reported the WARN_ON_ONCE(!tp->prior_cwnd) added in commit 8b8a321ff72c ("tcp: fix zero cwnd in tcp_cwnd_reduction") can trigger, and without a repro we would have to spend considerable time finding the bug.

Instead of complaining too late, we want to catch where and when tp->snd_cwnd is set to an illegal value.