selftests/bpf: add proper section name to bpf prog and rename it

test_skb_cgroup_id_kern.c is currently involved in a manual test. In its
current form, it can not be used with the auto-generated skeleton APIs,
because the section name is not valid to allow libbpf to deduce the program
type.

Update section name to allow skeleton APIs usage. Also rename the program
name to make it shorter and more straighforward regarding the API it is
testing. While doing so, make sure that test_skb_cgroup_id.sh passes to get
a working reference before converting it to test_progs
- update the obj name
- fix loading issue (verifier rejecting the program when loaded through tc,
  because of map not found), by preloading the whole obj with bpftool

Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20240813-convert_cgroup_tests-v4-3-a33c03458cf6@bootlin.com


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