[NETFILTER]: Use only 32bit counters for CONNTRACK_ACCT

Initially we used 64bit counters for conntrack-based accounting, since we
had no event mechanism to tell userspace that our counters are about to
overflow.  With nfnetlink_conntrack, we now have such a event mechanism and
thus can save 16bytes per connection.

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