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/ixgbevf/ixgbevf_main.c b/drivers/net/ixgbevf/ixgbevf_main.c
index 5d3c869..bdbd26c 100644
--- a/drivers/net/ixgbevf/ixgbevf_main.c
+++ b/drivers/net/ixgbevf/ixgbevf_main.c
@@ -356,7 +356,7 @@
 static inline void ixgbevf_rx_checksum(struct ixgbevf_adapter *adapter,
 				       u32 status_err, struct sk_buff *skb)
 {
-	skb->ip_summed = CHECKSUM_NONE;
+	skb_checksum_none_assert(skb);
 
 	/* Rx csum disabled */
 	if (!(adapter->flags & IXGBE_FLAG_RX_CSUM_ENABLED))