[NETFILTER]: nf_nat: add reference to conntrack from entry of bysource list

I will split 'struct nf_nat_info' out from conntrack. So I cannot use
'offsetof' to get the pointer to conntrack from it.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>