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/vmxnet3/vmxnet3_drv.c b/drivers/net/vmxnet3/vmxnet3_drv.c
index abe0ff5..198ce92 100644
--- a/drivers/net/vmxnet3/vmxnet3_drv.c
+++ b/drivers/net/vmxnet3/vmxnet3_drv.c
@@ -1042,11 +1042,11 @@
 				skb->csum = htons(gdesc->rcd.csum);
 				skb->ip_summed = CHECKSUM_PARTIAL;
 			} else {
-				skb->ip_summed = CHECKSUM_NONE;
+				skb_checksum_none_assert(skb);
 			}
 		}
 	} else {
-		skb->ip_summed = CHECKSUM_NONE;
+		skb_checksum_none_assert(skb);
 	}
 }