bpf: bpftool: Setting error code in do_loader()

missing error code in do_loader()
bpf_object__open_file() failed, but return 0
This means the command's exit status code was successful, so make sure to return the correct error code.

Link: https://lore.kernel.org/bpf/[email protected]/t/#u
Closes: libbpf/bpftool#156
Signed-off-by: nswon <[email protected]>

Author: nswon

tools/bpf/bpftool/prog.c

@@ -1928,6 +1928,7 @@ static int do_loader(int argc, char **argv)
 
 	obj = bpf_object__open_file(file, &open_opts);
 	if (!obj) {
+		err = libbpf_get_error(obj);
 		p_err("failed to open object file");
 		goto err_close_obj;
 	}