ipv4: Add helpers for neigh lookup for nexthop

A common theme in the output path is looking up a neigh entry for a
nexthop, either the gateway in an rtable or a fallback to the daddr
in the skb:

        nexthop = (__force u32)rt_nexthop(rt, ip_hdr(skb)->daddr);
        neigh = __ipv4_neigh_lookup_noref(dev, nexthop);
        if (unlikely(!neigh))
                neigh = __neigh_create(&arp_tbl, &nexthop, dev, false);

To allow the nexthop to be an IPv6 address we need to consider the
family of the nexthop and then call __ipv{4,6}_neigh_lookup_noref based
on it.

To make this simpler, add a ip_neigh_gw4 helper similar to ip_neigh_gw6
added in an earlier patch which handles:

        neigh = __ipv4_neigh_lookup_noref(dev, nexthop);
        if (unlikely(!neigh))
                neigh = __neigh_create(&arp_tbl, &nexthop, dev, false);

And then add a second one, ip_neigh_for_gw, that calls either
ip_neigh_gw4 or ip_neigh_gw6 based on the address family of the gateway.

Update the output paths in the VRF driver and core v4 code to use
ip_neigh_for_gw simplifying the family based lookup and making both
ready for a v6 nexthop.

ipv4_neigh_lookup has a different need - the potential to resolve a
passed in address in addition to any gateway in the rtable or skb. Since
this is a one-off, add ip_neigh_gw4 and ip_neigh_gw6 diectly. The
difference between __neigh_create used by the helpers and neigh_create
called by ipv4_neigh_lookup is taking a refcount, so add rcu_read_lock_bh
and bump the refcnt on the neigh entry.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/route.h

@@ -29,6 +29,8 @@
 #include <net/flow.h>
 #include <net/inet_sock.h>
 #include <net/ip_fib.h>
+#include <net/arp.h>
+#include <net/ndisc.h>
 #include <linux/in_route.h>
 #include <linux/rtnetlink.h>
 #include <linux/rcupdate.h>
@@ -350,4 +352,34 @@ static inline int ip4_dst_hoplimit(const struct dst_entry *dst)
 	return hoplimit;
 }
 
+static inline struct neighbour *ip_neigh_gw4(struct net_device *dev,
+					     __be32 daddr)
+{
+	struct neighbour *neigh;
+
+	neigh = __ipv4_neigh_lookup_noref(dev, daddr);
+	if (unlikely(!neigh))
+		neigh = __neigh_create(&arp_tbl, &daddr, dev, false);
+
+	return neigh;
+}
+
+static inline struct neighbour *ip_neigh_for_gw(struct rtable *rt,
+						struct sk_buff *skb,
+						bool *is_v6gw)
+{
+	struct net_device *dev = rt->dst.dev;
+	struct neighbour *neigh;
+
+	if (likely(rt->rt_gw_family == AF_INET)) {
+		neigh = ip_neigh_gw4(dev, rt->rt_gw4);
+	} else if (rt->rt_gw_family == AF_INET6) {
+		neigh = ip_neigh_gw6(dev, &rt->rt_gw6);
+		*is_v6gw = true;
+	} else {
+		neigh = ip_neigh_gw4(dev, ip_hdr(skb)->daddr);
+	}
+	return neigh;
+}
+
 #endif	/* _ROUTE_H */