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/e1000e/netdev.c b/drivers/net/e1000e/netdev.c
index 5f3eac6..c9b66f4 100644
--- a/drivers/net/e1000e/netdev.c
+++ b/drivers/net/e1000e/netdev.c
@@ -475,7 +475,8 @@
 {
 	u16 status = (u16)status_err;
 	u8 errors = (u8)(status_err >> 24);
-	skb->ip_summed = CHECKSUM_NONE;
+
+	skb_checksum_none_assert(skb);
 
 	/* Ignore Checksum bit is set */
 	if (status & E1000_RXD_STAT_IXSM)