[NETFILTER]: x_tables: error if ip_conntrack is asked to handle IPv6 packets

To do that, this makes nf_ct_l3proto_try_module_{get,put} compatible
functions. As a result we can remove '#ifdef' surrounds and direct call of
need_conntrack().

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>