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/qla3xxx.c b/drivers/net/qla3xxx.c
index 6168a13..7496ed2 100644
--- a/drivers/net/qla3xxx.c
+++ b/drivers/net/qla3xxx.c
@@ -2029,7 +2029,7 @@
 			 dma_unmap_len(lrg_buf_cb2, maplen),
 			 PCI_DMA_FROMDEVICE);
 	prefetch(skb->data);
-	skb->ip_summed = CHECKSUM_NONE;
+	skb_checksum_none_assert(skb);
 	skb->protocol = eth_type_trans(skb, qdev->ndev);
 
 	netif_receive_skb(skb);
@@ -2076,7 +2076,7 @@
 			 PCI_DMA_FROMDEVICE);
 	prefetch(skb2->data);
 
-	skb2->ip_summed = CHECKSUM_NONE;
+	skb_checksum_none_assert(skb2);
 	if (qdev->device_id == QL3022_DEVICE_ID) {
 		/*
 		 * Copy the ethhdr from first buffer to second. This