[NETFILTER]: nf_conntrack: attach conntrack to TCP RST generated by ip6t_REJECT

TCP RSTs generated by the REJECT target should be associated with the
conntrack of the original TCP packet. Since the conntrack entry is
usually not is the hash tables, it must be manually attached.

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>