selftests/bpf: Test case for BPF_SOCK_OPS_TCP_LISTEN_CB

selftests/bpf: Test case for BPF_SOCK_OPS_TCP_LISTEN_CB

Cover new TCP-BPF callback in test_tcpbpf: when listen() is called on
socket, set BPF_SOCK_OPS_STATE_CB_FLAG so that BPF_SOCK_OPS_STATE_CB
callback can be called on future state transition, and when such a
transition happens (TCP_LISTEN -> TCP_CLOSE), track it in the map and
verify it in user space later.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>