user operations errors should be properly converted to network byte order

Currently all errors are set like this:

```
reply.error = aop->write(chunk, len, from, userdata);
```

which requires to use `ntohl` inside user `buse_operations`.

I suggest to handle byte order on BUSE side, e.g.:

```
reply.error = htonl(aop->write(chunk, len, from, userdata));
```
