[NET]: Fix NULL pointer deref in checksum debugging.

The problem I was seeing turned out to be that skb->dev is NULL when
the checksum is being completed in user context. This happens because
the reference to the device is dropped (to allow it to be released
when packets are in the queue).

Because skb->dev was NULL, the netdev_rx_csum_fault was panicing on
deref of dev->name. How about this?

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>