Revert "net: Handle CHECKSUM_COMPLETE more adequately in pskb_trim_rcsum()."

This reverts commit 018c5bba.

It causes regressions for people using chips driven by the sungem
driver.  Suspicion is that the skb->csum value isn't being adjusted
properly.

The change also has a bug in that if __pskb_trim() fails, we'll leave
a corruped skb->csum value in there.  We would really need to revert
it to it's original value in that case.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -2263,6 +2263,24 @@ static inline void skb_postpull_rcsum(struct sk_buff *skb,
 
 unsigned char *skb_pull_rcsum(struct sk_buff *skb, unsigned int len);
 
+/**
+ *	pskb_trim_rcsum - trim received skb and update checksum
+ *	@skb: buffer to trim
+ *	@len: new length
+ *
+ *	This is exactly the same as pskb_trim except that it ensures the
+ *	checksum of received packets are still valid after the operation.
+ */
+
+static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
+{
+	if (likely(len >= skb->len))
+		return 0;
+	if (skb->ip_summed == CHECKSUM_COMPLETE)
+		skb->ip_summed = CHECKSUM_NONE;
+	return __pskb_trim(skb, len);
+}
+
 #define skb_queue_walk(queue, skb) \
 		for (skb = (queue)->next;					\
 		     skb != (struct sk_buff *)(queue);				\
@@ -2360,27 +2378,6 @@ __wsum __skb_checksum(const struct sk_buff *skb, int offset, int len,
 __wsum skb_checksum(const struct sk_buff *skb, int offset, int len,
 		    __wsum csum);
 
-/**
- *	pskb_trim_rcsum - trim received skb and update checksum
- *	@skb: buffer to trim
- *	@len: new length
- *
- *	This is exactly the same as pskb_trim except that it ensures the
- *	checksum of received packets are still valid after the operation.
- */
-
-static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
-{
-	if (likely(len >= skb->len))
-		return 0;
-	if (skb->ip_summed == CHECKSUM_COMPLETE) {
-		__wsum adj = skb_checksum(skb, len, skb->len - len, 0);
-
-		skb->csum = csum_sub(skb->csum, adj);
-	}
-	return __pskb_trim(skb, len);
-}
-
 static inline void *skb_header_pointer(const struct sk_buff *skb, int offset,
 				       int len, void *buffer)
 {