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/r8169.c b/drivers/net/r8169.c
index 078bbf4..07b3fb5 100644
--- a/drivers/net/r8169.c
+++ b/drivers/net/r8169.c
@@ -4460,7 +4460,7 @@
 	    ((status == RxProtoIP) && !(opts1 & IPFail)))
 		skb->ip_summed = CHECKSUM_UNNECESSARY;
 	else
-		skb->ip_summed = CHECKSUM_NONE;
+		skb_checksum_none_assert(skb);
 }
 
 static inline bool rtl8169_try_rx_copy(struct sk_buff **sk_buff,