drivers/net: avoid some skb->ip_summed initializations

fresh skbs have ip_summed set to CHECKSUM_NONE (0)

We can avoid setting again skb->ip_summed to CHECKSUM_NONE in drivers.

Introduce skb_checksum_none_assert() helper so that we keep this
assertion documented in driver sources.

Change most occurrences of :

skb->ip_summed = CHECKSUM_NONE;

by :

skb_checksum_none_assert(skb);

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/cpmac.c b/drivers/net/cpmac.c
index 5a5af1c..fec939f 100644
--- a/drivers/net/cpmac.c
+++ b/drivers/net/cpmac.c
@@ -391,7 +391,7 @@
 	if (likely(skb)) {
 		skb_put(desc->skb, desc->datalen);
 		desc->skb->protocol = eth_type_trans(desc->skb, priv->dev);
-		desc->skb->ip_summed = CHECKSUM_NONE;
+		skb_checksum_none_assert(desc->skb);
 		priv->dev->stats.rx_packets++;
 		priv->dev->stats.rx_bytes += desc->datalen;
 		result = desc->skb;