selftests/bpf: test_xdp_redirect: Rename BPF sections

selftests/bpf: test_xdp_redirect: Rename BPF sections

SEC("redirect_to_111") and SEC("redirect_to_222") can't be loaded by the
__load() helper.

Rename both sections SEC("xdp") so it can be interpreted by the __load()
helper in upcoming patch.
Update the test_xdp_redirect.sh to use the program name instead of the
section name to load the BPF program.

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://patch.msgid.link/20250110-xdp_redirect-v2-1-b8f3ae53e894@bootlin.com