selftests/bpf: test_xdp_veth: Rename config[]

selftests/bpf: test_xdp_veth: Rename config[]

The network topology is held by the config[] table. This 'config' name
is a bit too generic if we want to add other configuration variables.

Rename config[] to net_config[].

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250131-redirect-multi-v4-6-970b33678512@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>