tun: tun_init_nat always uses TUN_MTU_DEFAULT (1500), ignore ETCP mtu from config

src/tun_if.c

@@ -223,7 +223,8 @@ fail:
 struct tun_if* tun_init_nat(struct UASYNC* ua, const char* ifname, const char* ip_addr_str, int mtu, int test_mode)
 {
     if (!ua || !ifname || !ip_addr_str) return NULL;
-    if (mtu <= 0) mtu = TUN_MTU_DEFAULT;
+    (void)mtu;
+    int tun_mtu = TUN_MTU_DEFAULT;
 
     struct tun_if* tun = u_calloc(1, sizeof(struct tun_if));
     if (!tun) return NULL;
@@ -238,7 +239,7 @@ struct tun_if* tun_init_nat(struct UASYNC* ua, const char* ifname, const char* i
     snprintf(ip_str, sizeof(ip_str), "%s/32", ip_addr_str);
 
     if (!test_mode) {
-        if (tun_platform_init(tun, tun->ifname, ip_str, mtu) != 0) {
+        if (tun_platform_init(tun, tun->ifname, ip_str, tun_mtu) != 0) {
             u_free(tun);
             return NULL;
         }