selftests/bpf: remove duplicated functions

The file cgroup_tcp_skb.c contains redundant implementations of the similar
functions (create_server_sock_v6(), connect_client_server_v6() and
get_sock_port_v6()) found in network_helpers.c. Let's eliminate these
duplicated functions.

Changes from v1:

- Remove get_sock_port_v6() as well.

v1: https://lore.kernel.org/all/20230807193840.567962-1-thinker.li@gmail.com/

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20230808162858.326871-1-thinker.li@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

selftests/bpf: fix the incorrect verification of port numbers.

Check port numbers before calling htons().

According to Dan Carpenter's report, Smatch identified incorrect port
number checks. It is expected that the returned port number is an integer,
with negative numbers indicating errors. However, the value was mistakenly
verified after being translated by htons().

Major changes from v1:

- Move the variable 'port' to the same line of 'err'.

Fixes: 539c7e67aa4a ("selftests/bpf: Verify that the cgroup_skb filters receive expected packets.")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/bpf/cafd6585-d5a2-4096-b94f-7556f5aa7737@moroto.mountain/
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20230804165831.173627-1-thinker.li@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

selftests/bpf: Verify that the cgroup_skb filters receive expected packets.

This test case includes four scenarios:

1. Connect to the server from outside the cgroup and close the connection
from outside the cgroup.
2. Connect to the server from outside the cgroup and close the connection
from inside the cgroup.
3. Connect to the server from inside the cgroup and close the connection
from outside the cgroup.
4. Connect to the server from inside the cgroup and close the connection
from inside the cgroup.

The test case is to verify that cgroup_skb/{egress, ingress} filters
receive expected packets including SYN, SYN/ACK, ACK, FIN, and FIN/ACK.

Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230624014600.576756-3-kuifeng@meta.com