net: Add skb_unclone() helper function.

This function will be used in next GRE_GSO patch. This patch does
not change any functionality.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Eric Dumazet <edumazet@google.com>
diff --git a/net/sched/act_ipt.c b/net/sched/act_ipt.c
index 0fb9e3f..e0f6de6 100644
--- a/net/sched/act_ipt.c
+++ b/net/sched/act_ipt.c
@@ -207,10 +207,8 @@
 	struct tcf_ipt *ipt = a->priv;
 	struct xt_action_param par;
 
-	if (skb_cloned(skb)) {
-		if (pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
-			return TC_ACT_UNSPEC;
-	}
+	if (skb_unclone(skb, GFP_ATOMIC))
+		return TC_ACT_UNSPEC;
 
 	spin_lock(&ipt->tcf_lock);