selftests/bpf: netns_new() and netns_free() helpers.

selftests/bpf: netns_new() and netns_free() helpers.

[ Upstream commit 1e115a58be0ffca63727dc0495dae924a19f8cd4 ]

netns_new()/netns_free() create/delete network namespaces. They support the
option '-m' of test_progs to start/stop traffic monitor for the network
namespace being created for matched tests.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240815053254.470944-4-thinker.li@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Stable-dep-of: 5bf1557e3d6a ("selftests/bpf: Fix backtrace printing for selftests crashes")
Signed-off-by: Sasha Levin <sashal@kernel.org>