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

In the Linux kernel, the following vulnerability has been resolved: tcp: add accessors to read/set tp-sndcwnd We had various bugs over the years with code breaking the assumption that tp-sndcwnd is greater than zero. Lately, syzbot reported the WARNONONCE!tp-priorcwnd added in commit 8b8a321ff72c...