ipv4: fix forwarding for strict source routes

After the change "Adjust semantics of rt->rt_gateway"
(commit f8126f1d51) rt_gateway can be 0 but ip_forward() compares
it directly with nexthop. What we want here is to check if traffic
is to directly connected nexthop and to fail if using gateway.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>