ipv4: move fib4_has_custom_rules() helper to public header

So that we can use it in the next patch.
Additionally constify the helper argument.
Suggested-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Reviewed-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/ip_fib.h

@@ -311,6 +311,11 @@ static inline int fib_lookup(struct net *net, const struct flowi4 *flp,
 	return err;
 }
 
+static inline bool fib4_has_custom_rules(const struct net *net)
+{
+	return false;
+}
+
 static inline bool fib4_rule_default(const struct fib_rule *rule)
 {
 	return true;
@@ -378,6 +383,11 @@ static inline int fib_lookup(struct net *net, struct flowi4 *flp,
 	return err;
 }
 
+static inline bool fib4_has_custom_rules(const struct net *net)
+{
+	return net->ipv4.fib_has_custom_rules;
+}
+
 bool fib4_rule_default(const struct fib_rule *rule);
 int fib4_rules_dump(struct net *net, struct notifier_block *nb,
 		    struct netlink_ext_ack *extack);

net/ipv4/fib_frontend.c

@@ -70,11 +70,6 @@ static int __net_init fib4_rules_init(struct net *net)
 	fib_free_table(main_table);
 	return -ENOMEM;
 }
-
-static bool fib4_has_custom_rules(struct net *net)
-{
-	return false;
-}
 #else
 
 struct fib_table *fib_new_table(struct net *net, u32 id)
@@ -131,11 +126,6 @@ struct fib_table *fib_get_table(struct net *net, u32 id)
 	}
 	return NULL;
 }
-
-static bool fib4_has_custom_rules(struct net *net)
-{
-	return net->ipv4.fib_has_custom_rules;
-}
 #endif /* CONFIG_IP_MULTIPLE_TABLES */
 
 static void fib_replace_table(struct net *net, struct fib_table *old,