[NETFILTER]: nf_conntrack: move registration of __nf_ct_attach

Move registration of __nf_ct_attach to nf_conntrack_core to make it usable
for IPv6 connection tracking as well.

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>