ipv4: Define __ipv4_neigh_lookup_noref when CONFIG_INET is disabled

BugLink: https://bugs.launchpad.net/bugs/1832661

commit 9b3040a6 upstream.

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>
Cc: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

include/net/arp.h

@@ -17,6 +17,7 @@ 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))
@@ -24,6 +25,13 @@ 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)
 {