bpf: Fix request_sock leak in sk lookup helpers

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

bpf: Fix request_sock leak in sk lookup helpers

A customer reported a request_socket 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 request_socket but returning the parent

LISTEN socket via sk_to_full_sk() without decrementing the child request socket

1st, resulting in request_sock slab object leak. This patch retains the

existing behaviour of returning full socks to the caller but it also decrements

the child request_socket 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 bpf_sk_release()

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.