Redesign struct hostapd_ip_addr to be of fixed size

This structure is embedded in some other structures and as such, it
would be nicer if this would not change its length based on build
options.

src/utils/ip_addr.h

@@ -16,13 +16,14 @@
 #define IP_ADDR_H
 
 struct hostapd_ip_addr {
+	int af; /* AF_INET / AF_INET6 */
 	union {
 		struct in_addr v4;
 #ifdef CONFIG_IPV6
 		struct in6_addr v6;
 #endif /* CONFIG_IPV6 */
+		u8 max_len[16];
 	} u;
-	int af; /* AF_INET / AF_INET6 */
 };
 
 const char * hostapd_ip_txt(const struct hostapd_ip_addr *addr, char *buf,