ip6_vti: fix creating fallback tunnel device for vti6

When set fb_tunnels_only_for_init_net to 1, don't create fallback tunnel
device for vti6 when a new namespace is created.

Tested:
[root@builder2 ~]# modprobe ip6_tunnel
[root@builder2 ~]# modprobe ip6_vti
[root@builder2 ~]# echo 1 > /proc/sys/net/core/fb_tunnels_only_for_init_net
[root@builder2 ~]# unshare -n
[root@builder2 ~]# ip link
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN mode DEFAULT group
default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_vti.c

@@ -1106,6 +1106,8 @@ static int __net_init vti6_init_net(struct net *net)
 	ip6n->tnls[0] = ip6n->tnls_wc;
 	ip6n->tnls[1] = ip6n->tnls_r_l;
 
+	if (!net_has_fallback_tunnels(net))
+		return 0;
 	err = -ENOMEM;
 	ip6n->fb_tnl_dev = alloc_netdev(sizeof(struct ip6_tnl), "ip6_vti0",
 					NET_NAME_UNKNOWN, vti6_dev_setup);