net/sched: act_ct: add nat mangle action only for NAT-conntrack

Currently add nat mangle action with comparing invert and orig tuple.
It is better to check IPS_NAT_MASK flags first to avoid non necessary
memcmp for non-NAT conntrack.

Signed-off-by: wenxu <wenxu@ucloud.cn>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>