netfilter: nf_conntrack_acct: use skb->len for accounting

use skb->len for accounting as xt_quota does. Since nf_conntrack works
at the network layer, skb_network_offset should always returns ZERO.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>