selftests/bpf: add ipv4 and dual ipv4/ipv6 support in btf_skc_cls_ingress

btf_skc_cls_ingress test currently checks that syncookie and
bpf_sk_assign/release helpers behave correctly in multiple scenarios,
but only with ipv6 socket.

Increase those helpers coverage by adding testing support for IPv4
sockets and IPv4/IPv6 sockets. The rework is mostly based on features
brought earlier in test_tcp_check_syncookie.sh to cover some fixes
performed on those helpers, but test_tcp_check_syncookie.sh is not
integrated in test_progs. The most notable changes linked to this are:
- some rework in the corresponding eBPF program to support both types of
  traffic
- the switch from start_server to start_server_str to allow to check
  some socket options
- the introduction of new subtests for ipv4 and ipv4/ipv6

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20241020-syncookie-v2-4-2db240225fed@bootlin.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>