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/jme.c b/drivers/net/jme.c
index 99f24f5..1fcd533 100644
--- a/drivers/net/jme.c
+++ b/drivers/net/jme.c
@@ -936,7 +936,7 @@
 		if (jme_rxsum_ok(jme, le16_to_cpu(rxdesc->descwb.flags)))
 			skb->ip_summed = CHECKSUM_UNNECESSARY;
 		else
-			skb->ip_summed = CHECKSUM_NONE;
+			skb_checksum_none_assert(skb);
 
 		if (rxdesc->descwb.flags & cpu_to_le16(RXWBFLAG_TAGON)) {
 			if (jme->vlgrp) {