[PATCH v3 bpf-next] selftests/bpf: Fix the issue where the error code is 0

[Feng Yang <yangfeng59949@163.com>]

From: Feng Yang <yangfeng@kylinos.cn>

The error message printed here only uses the previous err value,
which results in it being printed as 0.
When bpf_map__attach_struct_ops encounters an error,
it uses libbpf_err_ptr(err) to set errno = -err and returns NULL.
Therefore, Using -errno can fix this issue.

Fix before:
run_subtest:FAIL:1019 bpf_map__attach_struct_ops failed for map pro_epilogue: err=0

Fix after:
run_subtest:FAIL:1019 bpf_map__attach_struct_ops failed for map pro_epilogue: err=-9

Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
---
Changes in v3:
- Use -errno here directly, thanks: Andrii Nakryiko.
- Link to v2: https://lore.kernel.org/all/20250829014125.198653-1-yangfeng59949@163.com/
---
Changes in v2:
- Use libbpf_get_error, thanks: Alexei Starovoitov.
- Link to v1: https://lore.kernel.org/all/20250828081507.1380218-1-yangfeng59949@163.com/
---
 tools/testing/selftests/bpf/test_loader.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/test_loader.c b/tools/testing/selftests/bpf/test_loader.c
index 78423cf89e01..33d59c093a27 100644
--- a/tools/testing/selftests/bpf/test_loader.c
+++ b/tools/testing/selftests/bpf/test_loader.c
@@ -1083,7 +1083,7 @@ void run_subtest(struct test_loader *tester,
 			link = bpf_map__attach_struct_ops(map);
 			if (!link) {
 				PRINT_FAIL("bpf_map__attach_struct_ops failed for map %s: err=%d\n",
-					   bpf_map__name(map), err);
+					   bpf_map__name(map), -errno);
 				goto tobj_cleanup;
 			}
 			links[links_cnt++] = link;
-- 
2.25.1