selftests/bpf: add missing ns cleanups in btf_skc_cls_ingress

selftests/bpf: add missing ns cleanups in btf_skc_cls_ingress

btf_skc_cls_ingress.c currently runs two subtests, and create a
dedicated network namespace for each, but never cleans up the created
namespace once the test has ended.

Add missing namespace cleanup after each subtest to avoid accumulating
namespaces for each new subtest. While at it, switch namespace
management to netns_{new,free}

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