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

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/3308676ec525901bf1656014003c443a60730a04
• https://git.kernel.org/stable/c/5aba0ad44fb4a7fb78c5076c313456de199a3c29
• https://git.kernel.org/stable/c/41e191fe72282e193a7744e2fc1786b23156c9e4
• https://git.kernel.org/stable/c/40570375356c874b1578e05c1dcc3ff7c1322dbe