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/atl1c/atl1c_main.c b/drivers/net/atl1c/atl1c_main.c
index 61f1634..553230e 100644
--- a/drivers/net/atl1c/atl1c_main.c
+++ b/drivers/net/atl1c/atl1c_main.c
@@ -1719,7 +1719,7 @@
 	 * cannot figure out if the packet is fragmented or not,
 	 * so we tell the KERNEL CHECKSUM_NONE
 	 */
-	skb->ip_summed = CHECKSUM_NONE;
+	skb_checksum_none_assert(skb);
 }
 
 static int atl1c_alloc_rx_buffer(struct atl1c_adapter *adapter, const int ringid)