net: Handle CHECKSUM_COMPLETE more adequately in pskb_trim_rcsum().

Currently pskb_trim_rcsum() just balks on CHECKSUM_COMPLETE packets
and remarks them as CHECKSUM_NONE, forcing a software checksum
validation later.

We have all of the mechanics available to fixup the skb->csum value,
even for complicated fragmented packets, via the helpers
skb_checksum() and csum_sub().

So just use them.

Based upon a suggestion by Herbert Xu.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -2263,24 +2263,6 @@ 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);				\
@@ -2378,6 +2360,27 @@ __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)
 {