[NET_CLS_ACT]: Introduce skb_act_clone

Reworked skb_clone looks uglier with the single ifdef
CONFIG_NET_CLS_ACT This patch introduces skb_act_clone which will
replace skb_clone in tc actions

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