selftests/bpf: ensure proper root namespace cleanup when test fail

serial_test_flow_dissector_namespace manipulates both the root net
namespace and a dedicated non-root net namespace. If for some reason a
program attach on root namespace succeeds while it was expected to
fail, the unexpected program will remain attached to the root namespace,
possibly affecting other runs or even other tests in the same run.

Fix undesired test failure side effect by explicitly detaching programs
on failing tests expecting attach to fail. As a side effect of this
change, do not test errno value if the tested operation do not fail.

Fixes: 284ed00a59dd ("selftests/bpf: migrate flow_dissector namespace exclusivity test")
Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://lore.kernel.org/r/20241128-small_flow_test_fix-v1-1-c12d45c98c59@bootlin.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>