[NETFILTER]: Add new iptables "connbytes" match

This patch ads a new "connbytes" match that utilizes the CONFIG_NF_CT_ACCT
per-connection byte and packet counters.  Using it you can do things like
packet classification on average packet size within a connection.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>