CVE-2023-54226 af_unix: Fix data races around sk->sk_shutdown.

In the Linux kernel, the following vulnerability has been resolved: afunix: Fix data races around sk-skshutdown. KCSAN found a data race around sk-skshutdown where unixreleasesock and unixshutdown update it under unixstatelock, OTOH unixpoll and unixdgrampoll read it locklessly. We need to annota...