[PATCH v2 bpf-next] selftests/bpf: Fix the issue where the error code is 0

[Feng Yang]

From: Feng Yang <yangfeng@xxxxxxxxxx>

The error message printed here only uses the previous err value,
which results in it being printed as 0.
Fix this issue by using libbpf_get_error to retrieve the error.

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@xxxxxxxxxx>
---
Changes in v2:
- Use libbpf_get_error, thanks: Alexei Starovoitov.
- Link to v1: https://lore.kernel.org/all/20250828081507.1380218-1-yangfeng59949@xxxxxxx/
---
 tools/testing/selftests/bpf/test_loader.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/test_loader.c b/tools/testing/selftests/bpf/test_loader.c
index 78423cf89e01..b8e102eb0908 100644
--- a/tools/testing/selftests/bpf/test_loader.c
+++ b/tools/testing/selftests/bpf/test_loader.c
@@ -1082,8 +1082,8 @@ 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);
+				PRINT_FAIL("bpf_map__attach_struct_ops failed for map %s: err=%ld\n",
+					   bpf_map__name(map), libbpf_get_error(link));
 				goto tobj_cleanup;
 			}
 			links[links_cnt++] = link;
-- 
2.25.1