[NETFILTER]: Make netfilter code use the seq_open_private

Just switch to the consolidated calls.

ipt_recent() has to initialize the private, so use
the __seq_open_private() helper.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>