netfilter: conntrack: avoid zeroing timer

add a __nfct_init_offset annotation member to struct nf_conn to make
it clear which members are covered by the memset when the conntrack
is allocated.

This avoids zeroing timer_list and ct_net; both are already inited
explicitly.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>