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/ixgbe/ixgbe_fcoe.c b/drivers/net/ixgbe/ixgbe_fcoe.c
index 86fa07c..2f1de8b 100644
--- a/drivers/net/ixgbe/ixgbe_fcoe.c
+++ b/drivers/net/ixgbe/ixgbe_fcoe.c
@@ -304,12 +304,13 @@
 	if (!ixgbe_rx_is_fcoe(rx_desc))
 		goto ddp_out;
 
-	skb->ip_summed = CHECKSUM_UNNECESSARY;
 	sterr = le32_to_cpu(rx_desc->wb.upper.status_error);
 	fcerr = (sterr & IXGBE_RXDADV_ERR_FCERR);
 	fceofe = (sterr & IXGBE_RXDADV_ERR_FCEOFE);
 	if (fcerr == IXGBE_FCERR_BADCRC)
-		skb->ip_summed = CHECKSUM_NONE;
+		skb_checksum_none_assert(skb);
+	else
+		skb->ip_summed = CHECKSUM_UNNECESSARY;
 
 	if (eth_hdr(skb)->h_proto == htons(ETH_P_8021Q))
 		fh = (struct fc_frame_header *)(skb->data +