BPF exceeds default net.core.optmem_max limit

If you try to run tests / use the filter on stock Ubuntu, you get the following error:

```
$ sudo -E go test
--- FAIL: TestNew (0.08s)
rakelimit_test.go:14: Can't create limiter: can't attach BPF to socket: cannot allocate memory
```

This is because `SO_ATTACH_BPF` checks the size of the program against `net.core.optmem_max limit`. The default value for that on my Ubuntu install is `20480`. We currently need something > 32768 but < 65536.

We should try to get the filter size below the default value. See also #2.