selftests/bpf: Fix build errors if CONFIG_NF_CONNTRACK=m

selftests/bpf: Fix build errors if CONFIG_NF_CONNTRACK=m

If CONFIG_NF_CONNTRACK=m, there are no definitions of NF_NAT_MANIP_SRC
and NF_NAT_MANIP_DST in vmlinux.h, build test_bpf_nf.c failed.

$ make -C tools/testing/selftests/bpf/

  CLNG-BPF [test_maps] test_bpf_nf.bpf.o
progs/test_bpf_nf.c:160:42: error: use of undeclared identifier 'NF_NAT_MANIP_SRC'
                bpf_ct_set_nat_info(ct, &saddr, sport, NF_NAT_MANIP_SRC);
                                                       ^
progs/test_bpf_nf.c:163:42: error: use of undeclared identifier 'NF_NAT_MANIP_DST'
                bpf_ct_set_nat_info(ct, &daddr, dport, NF_NAT_MANIP_DST);
                                                       ^
2 errors generated.

Copy the definitions in include/net/netfilter/nf_nat.h to test_bpf_nf.c,
in order to avoid redefinitions if CONFIG_NF_CONNTRACK=y, rename them with
___local suffix. This is similar with commit 1058b6a78db2 ("selftests/bpf:
Do not fail build if CONFIG_NF_CONNTRACK=m/n").

Fixes: b06b45e82b59 ("selftests/bpf: add tests for bpf_ct_set_nat_info kfunc")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/1674028604-7113-1-git-send-email-yangtiezhu@loongson.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>