bcc: Fix vmlinux kfunc load

The bpf_attr::attach_btf_obj_fd is set to -1
when loading prog for vmlinux kfunc/kretfunc,
which results in EINVAL. Set it to 0 instead.

Closes iovisor#4607.

Signed-off-by: Hengqi Chen <[email protected]>

src/cc/libbpf.c

@@ -862,7 +862,7 @@ int bcc_prog_load_xattr(enum bpf_prog_type prog_type, const char *prog_name,
  return ret;
  }
 
- opts->attach_btf_obj_fd = fd;
+ opts->attach_btf_obj_fd = fd == -1 ? 0 : fd;
  opts->attach_btf_id = ret;
  opts->expected_attach_type = expected_attach_type;
  }