bugfix: ngx-accept-queue: fix max accept queue limit

I think the `max_syn_qlen` should be `1 << max_qlen_log`.
In kernel-2.6.18:
```c
    for (lopt->max_qlen_log = 6;
         (1 << lopt->max_qlen_log) < sysctl_max_syn_backlog;
         lopt->max_qlen_log++);
```

In kernel-2.6.32:
```c
   for (lopt->max_qlen_log = 3;
         (1 << lopt->max_qlen_log) < nr_table_entries;
         lopt->max_qlen_log++);
```