net: factor out a helper to decrement the skb refcount

The same code is replicated in 3 different places; move it to a
common helper.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index e508c1e..747263c 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -694,12 +694,9 @@ EXPORT_SYMBOL(__kfree_skb);
  */
 void kfree_skb(struct sk_buff *skb)
 {
-	if (unlikely(!skb))
+	if (!skb_unref(skb))
 		return;
-	if (likely(atomic_read(&skb->users) == 1))
-		smp_rmb();
-	else if (likely(!atomic_dec_and_test(&skb->users)))
-		return;
+
 	trace_kfree_skb(skb, __builtin_return_address(0));
 	__kfree_skb(skb);
 }
@@ -746,12 +743,9 @@ EXPORT_SYMBOL(skb_tx_error);
  */
 void consume_skb(struct sk_buff *skb)
 {
-	if (unlikely(!skb))
+	if (!skb_unref(skb))
 		return;
-	if (likely(atomic_read(&skb->users) == 1))
-		smp_rmb();
-	else if (likely(!atomic_dec_and_test(&skb->users)))
-		return;
+
 	trace_consume_skb(skb);
 	__kfree_skb(skb);
 }