selftests/bpf: Update xdp_redirect_map prog sections for libbpf

xdp_redirect_map.c is a bpf program used by test_xdp_veth.sh, which is not
handled by the generic test runner (test_progs). To allow converting this
test to test_progs, the corresponding program must be updated to allow
handling it through skeletons generated by bpftool and libbpf.

Update programs section names to allow to manipulate those with libbpf.

Signed-off-by: Alexis Lothoré (eBPF Foundation) <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Stanislav Fomichev <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]
Signed-off-by: Andrii Nakryiko <[e-mail]>

tools/testing/selftests/bpf/progs/xdp_redirect_map.c

···
 	__uint(value_size, sizeof(int));
 } tx_port SEC(".maps");
 
-SEC("redirect_map_0")
+SEC("xdp")
 int xdp_redirect_map_0(struct xdp_md *xdp)
 {
 	return bpf_redirect_map(&tx_port, 0, 0);
 }
 
-SEC("redirect_map_1")
+SEC("xdp")
 int xdp_redirect_map_1(struct xdp_md *xdp)
 {
 	return bpf_redirect_map(&tx_port, 1, 0);
 }
 
-SEC("redirect_map_2")
+SEC("xdp")
 int xdp_redirect_map_2(struct xdp_md *xdp)
 {
 	return bpf_redirect_map(&tx_port, 2, 0);