[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/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c
index 0f604d2..da9cfe9 100644
--- a/net/8021q/vlan_dev.c
+++ b/net/8021q/vlan_dev.c
@@ -163,10 +163,8 @@
 	stats->rx_packets++;
 	stats->rx_bytes += skb->len;
 
-	skb_pull(skb, VLAN_HLEN); /* take off the VLAN header (4 bytes currently) */
-
-	/* Need to correct hardware checksum */
-	skb_postpull_rcsum(skb, vhdr, VLAN_HLEN);
+	/* Take off the VLAN header (4 bytes currently) */
+	skb_pull_rcsum(skb, VLAN_HLEN);
 
 	/* Ok, lets check to make sure the device (dev) we
 	 * came in on is what this VLAN is attached to.