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, 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>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250908060810.1054341-1-yangfeng59949@163.com