ipv4: Adjust semantics of rt->rt_gateway.

In order to allow prefixed routes, we have to adjust how rt_gateway
is set and interpreted.

The new interpretation is:

1) rt_gateway == 0, destination is on-link, nexthop is iph->daddr

2) rt_gateway != 0, destination requires a nexthop gateway

Abstract the fetching of the proper nexthop value using a new
inline helper, rt_nexthop(), as suggested by Joe Perches.

Signed-off-by: David S. Miller <davem@davemloft.net>
Tested-by: Vijay Subramanian <subramanian.vijay@gmail.com>
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index c38293f..a0124eb 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -475,8 +475,7 @@
 		return 1;
 	}
 
-	paddr = skb_rtable(skb)->rt_gateway;
-
+	paddr = rt_nexthop(skb_rtable(skb), ip_hdr(skb)->daddr);
 	if (arp_set_predefined(inet_addr_type(dev_net(dev), paddr), haddr,
 			       paddr, dev))
 		return 0;