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>

drivers/net/ppp/ppp_generic.c

@@ -1805,8 +1805,7 @@ ppp_receive_nonmp_frame(struct ppp *ppp, struct sk_buff *skb)
 		/* the filter instructions are constructed assuming
 		   a four-byte PPP header on each packet */
 		if (ppp->pass_filter || ppp->active_filter) {
-			if (skb_cloned(skb) &&
-			    pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
+			if (skb_unclone(skb, GFP_ATOMIC))
 				goto err;
 
 			*skb_push(skb, 2) = 0;