selftests/bpf: fix bpf_map_redirect call for cpu map test

selftests/bpf: fix bpf_map_redirect call for cpu map test

xdp_redir_prog currently redirects packets based on the entry at index 1
in cpu_map, but the corresponding test only manipulates the entry at
index 0. This does not really affect the test in its current form since
the program is detached before having the opportunity to execute, but it
needs to be fixed before being able improve the corresponding test (ie,
not only test attach/detach but also the redirect feature)

Fix this XDP program by making it redirect packets based on entry 0 in
cpu_map instead of entry 1.

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20241009-convert_xdp_tests-v3-1-51cea913710c@bootlin.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>