[NETFILTER]: conntrack: cleanup the conntrack ID initialization

Currently the first conntrack ID assigned is 2, use 1 instead.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>