act_mirred: cleanup

1. don't let go back using goto.
2. don't call skb_act_clone() until it is necessary.
3. one exit of the critical context.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>