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/e1000/e1000_main.c b/drivers/net/e1000/e1000_main.c
index 3e8ac4b..17f5867 100644
--- a/drivers/net/e1000/e1000_main.c
+++ b/drivers/net/e1000/e1000_main.c
@@ -3552,7 +3552,8 @@
 	struct e1000_hw *hw = &adapter->hw;
 	u16 status = (u16)status_err;
 	u8 errors = (u8)(status_err >> 24);
-	skb->ip_summed = CHECKSUM_NONE;
+
+	skb_checksum_none_assert(skb);
 
 	/* 82543 or newer only */
 	if (unlikely(hw->mac_type < e1000_82543)) return;