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/ipg.c b/drivers/net/ipg.c
index 72e3d2d..dc01980 100644
--- a/drivers/net/ipg.c
+++ b/drivers/net/ipg.c
@@ -1213,7 +1213,7 @@
 
 	skb_put(skb, framelen);
 	skb->protocol = eth_type_trans(skb, dev);
-	skb->ip_summed = CHECKSUM_NONE;
+	skb_checksum_none_assert(skb);
 	netif_rx(skb);
 	sp->rx_buff[entry] = NULL;
 }
@@ -1278,7 +1278,7 @@
 				jumbo->skb->protocol =
 				    eth_type_trans(jumbo->skb, dev);
 
-				jumbo->skb->ip_summed = CHECKSUM_NONE;
+				skb_checksum_none_assert(jumbo->skb);
 				netif_rx(jumbo->skb);
 			}
 		}
@@ -1476,7 +1476,7 @@
 			 * IP/TCP/UDP frame was received. Let the
 			 * upper layer decide.
 			 */
-			skb->ip_summed = CHECKSUM_NONE;
+			skb_checksum_none_assert(skb);
 
 			/* Hand off frame for higher layer processing.
 			 * The function netif_rx() releases the sk_buff