selftests/bpf/test_xdp_meta: use temp netns for testing

Use temp netns instead of hard code name for testing in case the
netns already exists.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20220125081717.1260849-7-liuhangbin@gmail.com


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