[IPV4]: Clear skb cb on IP input

when data arrives at IP through loopback (and possibly other devices).
So the field needs to be cleared before it confuses the route code.
This was seen when running netem over loopback, but there are probably
other device cases. Maybe this should go into stable?

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