使用mmap实现的netlink，比原来基于sendmsg实现，性能要强上不小。
http://lwn.net/Articles/512442/
下面是测试：

nfq recv: regular netlink I/O
mnl recv: mmap'ed netlink I/O
batch: number of batched verdicts

1400 byte UDP packets, 8 cores, NFQUEUE balancing using 4 queues, only
mmap'ed RX, regular TX, userspace running inline Snort with all rules
and preprocessors enabled:

nfq recv, batch 0  1250 MBit total rx
mnl recv, batch 0  1230 MBit total rx
nfq recv, batch 10 1590 MBit total rx
mnl recv, batch 10 1770 MBit total rx