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/bnx2x/bnx2x_cmn.c b/drivers/net/bnx2x/bnx2x_cmn.c
index da96d1a..0e4caf4 100644
--- a/drivers/net/bnx2x/bnx2x_cmn.c
+++ b/drivers/net/bnx2x/bnx2x_cmn.c
@@ -623,7 +623,7 @@
 			/* Set Toeplitz hash for a none-LRO skb */
 			bnx2x_set_skb_rxhash(bp, cqe, skb);
 
-			skb->ip_summed = CHECKSUM_NONE;
+			skb_checksum_none_assert(skb);
 			if (bp->rx_csum) {
 				if (likely(BNX2X_RX_CSUM_OK(cqe)))
 					skb->ip_summed = CHECKSUM_UNNECESSARY;