[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/drivers/s390/net/qeth_main.c b/drivers/s390/net/qeth_main.c
index 2882202..c0ee6d9 100644
--- a/drivers/s390/net/qeth_main.c
+++ b/drivers/s390/net/qeth_main.c
@@ -2306,7 +2306,7 @@
 	struct iphdr *ip_hdr;
 
 	QETH_DBF_TEXT(trace,5,"skbfktr");
-	skb->mac.raw = skb->data - QETH_FAKE_LL_LEN_TR;
+	skb_set_mac_header(skb, -QETH_FAKE_LL_LEN_TR);
 	/* this is a fake ethernet header */
 	fake_hdr = tr_hdr(skb);
 
@@ -2359,7 +2359,7 @@
 	struct iphdr *ip_hdr;
 
 	QETH_DBF_TEXT(trace,5,"skbfketh");
-	skb->mac.raw = skb->data - QETH_FAKE_LL_LEN_ETH;
+	skb_set_mac_header(skb, -QETH_FAKE_LL_LEN_ETH);
 	/* this is a fake ethernet header */
 	fake_hdr = eth_hdr(skb);