net: use CHECKSUM_NONE instead of magic number

Two places in the kernel were doing skb->ip_summed = 0.

Change both to skb->ip_summed = CHECKSUM_NONE, which is more readable.

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Signed-off-by: David S. Miller <davem@davemloft.net>