[SK_BUFF]: Introduce skb_set_mac_header()

For the cases where we want to set skb->mac.raw to an offset from skb->data.

Simple cases first, the memmove ones and specially pktgen will be left for later.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 748f254..43ab6cb 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -965,6 +965,11 @@
 	skb->mac.raw = skb->data;
 }
 
+static inline void skb_set_mac_header(struct sk_buff *skb, const int offset)
+{
+	skb->mac.raw = skb->data + offset;
+}
+
 /*
  * CPUs often take a performance hit when accessing unaligned memory
  * locations. The actual performance hit varies, it can be small if the