selftests/bpf: fix btf_dedup testing code

btf_dedup testing code doesn't account for length of struct btf_header
when calculating the start of a string section. This patch fixes this
problem.

Fixes: 49b57e0d ("tools/bpf: remove btf__get_strings() superseded by raw data API")
Signed-off-by: NAndrii Nakryiko <andriin@fb.com>
Acked-by: NSong Liu <songliubraving@fb.com>
Acked-by: NYonghong Song <yhs@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/.gitignore

@@ -14,6 +14,7 @@ feature
 test_libbpf_open
 test_sock
 test_sock_addr
+test_sock_fields
 urandom_read
 test_btf
 test_sockmap

tools/testing/selftests/bpf/test_btf.c

@@ -5936,9 +5936,9 @@ static int do_test_dedup(unsigned int test_num)
 	}
 
 	test_hdr = test_btf_data;
-	test_strs = test_btf_data + test_hdr->str_off;
+	test_strs = test_btf_data + sizeof(*test_hdr) + test_hdr->str_off;
 	expect_hdr = expect_btf_data;
-	expect_strs = expect_btf_data + expect_hdr->str_off;
+	expect_strs = expect_btf_data + sizeof(*test_hdr) + expect_hdr->str_off;
 	if (CHECK(test_hdr->str_len != expect_hdr->str_len,
 		  "test_hdr->str_len:%u != expect_hdr->str_len:%u",
 		  test_hdr->str_len, expect_hdr->str_len)) {