[NET]: Add missing memory barrier to kfree_skb().

Also kill kfree_skb_fast(), that is a relic from fast switching
which was killed off years ago.

The bug is that in the case where we do the atomic_read()
optimization, we need to make sure that reads of skb state
later in __kfree_skb() processing (particularly the skb->list
BUG check) are not reordered to occur before the counter
read by the cpu.

Thanks to Olaf Kirch and Anton Blanchard for discovering
and helping fix this bug.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -353,15 +353,11 @@ static inline struct sk_buff *skb_get(struct sk_buff *skb)
  */
 static inline void kfree_skb(struct sk_buff *skb)
 {
-	if (atomic_read(&skb->users) == 1 || atomic_dec_and_test(&skb->users))
-		__kfree_skb(skb);
-}
-
-/* Use this if you didn't touch the skb state [for fast switching] */
-static inline void kfree_skb_fast(struct sk_buff *skb)
-{
-	if (atomic_read(&skb->users) == 1 || atomic_dec_and_test(&skb->users))
-		kfree_skbmem(skb);
+	if (likely(atomic_read(&skb->users) == 1))
+		smp_rmb();
+	else if (likely(!atomic_dec_and_test(&skb->users)))
+		return;
+	__kfree_skb(skb);
 }
 
 /**