CVE-2022-49697: bpf: Fix request_sock leak in sk lookup helpers

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

bpf: Fix requestsock leak in sk lookup helpers

A customer reported a requestsocket leak in a Calico cloud environment. We found that a BPF program was doing a socket lookup with takes a refcnt on the socket and that it was finding the requestsocket but returning the parent LISTEN socket via sktofullsk() without decrementing the child request socket 1st, resulting in requestsock slab object leak. This patch retains the existing behaviour of returning full socks to the caller but it also decrements the child requestsocket if one is present before doing so to prevent the leak.

Thanks to Curtis Taylor for all the help in diagnosing and testing this. And thanks to Antoine Tenart for the reproducer and patch input.

v2 of this patch contains, refactor as per Daniel Borkmann's suggestions to validate RCU flags on the listen socket so that it balances with bpfskrelease() and update comments as per Martin KaFai Lau's suggestion. One small change to Daniels suggestion, put "sk = sk2" under "if (sk2 != sk)" to avoid an extra instruction.