ipv4: Set rt->rt_iif more sanely on output routes.

ipv4: Set rt->rt_iif more sanely on output routes.

rt->rt_iif is only ever inspected on input routes, for example DCCP
uses this to populate a route lookup flow key when generating replies
to another packet.

Therefore, setting it to anything other than zero on output routes
makes no sense.

Signed-off-by: David S. Miller <davem@davemloft.net>