[NET]: Replace skb_pull/skb_postpull_rcsum with skb_pull_rcsum

We're now starting to have quite a number of places that do skb_pull
followed immediately by an skb_postpull_rcsum.  We can merge these two
operations into one function with skb_pull_rcsum.  This makes sense
since most pull operations on receive skb's need to update the
checksum.

I've decided to make this out-of-line since it is fairly big and the
fast path where hardware checksums are enabled need to call
csum_partial anyway.

Since this is a brand new function we get to add an extra check on the
len argument.  As it is most callers of skb_pull ignore its return
value which essentially means that there is no check on the len
argument.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ppp_generic.c b/drivers/net/ppp_generic.c
index 0245e40..f608c12 100644
--- a/drivers/net/ppp_generic.c
+++ b/drivers/net/ppp_generic.c
@@ -1691,8 +1691,8 @@
 		    || ppp->npmode[npi] != NPMODE_PASS) {
 			kfree_skb(skb);
 		} else {
-			skb_pull(skb, 2);	/* chop off protocol */
-			skb_postpull_rcsum(skb, skb->data - 2, 2);
+			/* chop off protocol */
+			skb_pull_rcsum(skb, 2);
 			skb->dev = ppp->dev;
 			skb->protocol = htons(npindex_to_ethertype[npi]);
 			skb->mac.raw = skb->data;