selftests/bpf: Fix the issue where the error code is 0

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, strerror(errno) can be used to 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: Bad file descriptor

Signed-off-by: Feng Yang <[email protected]>

Author: kknjh

tools/testing/selftests/bpf/test_loader.c

@@ -1087,8 +1087,8 @@ 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: %s\n",
+					   bpf_map__name(map), strerror(errno));
 				goto tobj_cleanup;
 			}
 			links[links_cnt++] = link;