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: David Ahern <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>
Reviewed-by: David Ahern <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/ip_fib.h

···
 	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;
···
 	rcu_read_unlock();
 
 	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);

net/ipv4/fib_frontend.c

···
 	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)
···
 			return tb;
 	}
 	return NULL;
-}
-
-static bool fib4_has_custom_rules(struct net *net)
-{
-	return net->ipv4.fib_has_custom_rules;
 }
 #endif /* CONFIG_IP_MULTIPLE_TABLES */