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/tehuti.c b/drivers/net/tehuti.c
index 3128d6a..8b3dc1e 100644
--- a/drivers/net/tehuti.c
+++ b/drivers/net/tehuti.c
@@ -1297,12 +1297,13 @@
 		ndev->stats.rx_bytes += len;
 
 		skb_put(skb, len);
-		skb->ip_summed = CHECKSUM_UNNECESSARY;
 		skb->protocol = eth_type_trans(skb, ndev);
 
 		/* Non-IP packets aren't checksum-offloaded */
 		if (GET_RXD_PKT_ID(rxd_val1) == 0)
-			skb->ip_summed = CHECKSUM_NONE;
+			skb_checksum_none_assert(skb);
+		else
+			skb->ip_summed = CHECKSUM_UNNECESSARY;
 
 		NETIF_RX_MUX(priv, rxd_val1, rxd_vlan, skb);