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/benet/be_main.c b/drivers/net/benet/be_main.c
index 9db10fe..dcee7a2 100644
--- a/drivers/net/benet/be_main.c
+++ b/drivers/net/benet/be_main.c
@@ -1016,7 +1016,7 @@
 	skb_fill_rx_data(adapter, skb, rxcp, num_rcvd);
 
 	if (do_pkt_csum(rxcp, adapter->rx_csum))
-		skb->ip_summed = CHECKSUM_NONE;
+		skb_checksum_none_assert(skb);
 	else
 		skb->ip_summed = CHECKSUM_UNNECESSARY;