netfilter: conntrack: Use memset_startat() to zero struct nf_conn

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Use memset_startat() to avoid confusing memset() about writing beyond
the target struct member.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>