netfilter: conntrack: introduce nf_ct_acct_update()

Introduce a helper function to update conntrack counters.
__nf_ct_kill_acct() was unnecessarily subtracting skb_network_offset()
that is expected to be zero from the ipv4/ipv6 hooks.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>