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/macb.c b/drivers/net/macb.c
index ff2f158..4297f6e 100644
--- a/drivers/net/macb.c
+++ b/drivers/net/macb.c
@@ -407,7 +407,7 @@
 	}
 
 	skb_reserve(skb, RX_OFFSET);
-	skb->ip_summed = CHECKSUM_NONE;
+	skb_checksum_none_assert(skb);
 	skb_put(skb, len);
 
 	for (frag = first_frag; ; frag = NEXT_RX(frag)) {