ipv4: Define __ipv4_neigh_lookup_noref when CONFIG_INET is disabled

Define __ipv4_neigh_lookup_noref to return NULL when CONFIG_INET is disabled.

Fixes: 4b2a2bfe ("neighbor: Call __ipv4_neigh_lookup_noref in neigh_xmit")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/arp.h

@@ -18,6 +18,7 @@ static inline u32 arp_hashfn(const void *pkey, const struct net_device *dev, u32
 	return val * hash_rnd[0];
 }
 
+#ifdef CONFIG_INET
 static inline struct neighbour *__ipv4_neigh_lookup_noref(struct net_device *dev, u32 key)
 {
 	if (dev->flags & (IFF_LOOPBACK | IFF_POINTOPOINT))
@@ -25,6 +26,13 @@ static inline struct neighbour *__ipv4_neigh_lookup_noref(struct net_device *dev
 
 	return ___neigh_lookup_noref(&arp_tbl, neigh_key_eq32, arp_hashfn, &key, dev);
 }
+#else
+static inline
+struct neighbour *__ipv4_neigh_lookup_noref(struct net_device *dev, u32 key)
+{
+	return NULL;
+}
+#endif
 
 static inline struct neighbour *__ipv4_neigh_lookup(struct net_device *dev, u32 key)
 {