net/sched: act_ct: fix err check for nf_conntrack_confirm

The confirm operation should be checked. If there are any failed,
the packet should be dropped like in ovs and netfilter.

Fixes: b57dc7c1

 ("net/sched: Introduce action ct")
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>