selftests/bpf: test_xdp_veth: Use int to describe next veth

selftests/bpf: test_xdp_veth: Use int to describe next veth

In the struct veth_configuration, the next_veth string is used to tell
the next virtual interface to which packets must be redirected to. So it
has to match the local_veth string of an other veth_configuration.

Change next_veth type to int to avoid handling two identical strings.
This integer is used as an offset in the network configuration table.

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250131-redirect-multi-v4-4-970b33678512@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>