ip6_tunnel: fix IFLA_MTU ignored on NEWLINK

Commit 128bb975 ("ip6_gre: init dev->mtu and dev->hard_header_len
correctly") fixed IFLA_MTU ignored on NEWLINK for ip6_gre. The same
mtu fix is also needed for ip6_tunnel.

Note that dev->hard_header_len setting for ip6_tunnel works fine,
no need to fix it.
Reported-by: NJianlin Shi <jishi@redhat.com>
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_tunnel.c

@@ -1982,14 +1982,14 @@ static int ip6_tnl_newlink(struct net *src_net, struct net_device *dev,
 {
 	struct net *net = dev_net(dev);
 	struct ip6_tnl_net *ip6n = net_generic(net, ip6_tnl_net_id);
-	struct ip6_tnl *nt, *t;
 	struct ip_tunnel_encap ipencap;
+	struct ip6_tnl *nt, *t;
+	int err;
 
 	nt = netdev_priv(dev);
 
 	if (ip6_tnl_netlink_encap_parms(data, &ipencap)) {
-		int err = ip6_tnl_encap_setup(nt, &ipencap);
-
+		err = ip6_tnl_encap_setup(nt, &ipencap);
 		if (err < 0)
 			return err;
 	}
@@ -2005,7 +2005,11 @@ static int ip6_tnl_newlink(struct net *src_net, struct net_device *dev,
 			return -EEXIST;
 	}
 
-	return ip6_tnl_create2(dev);
+	err = ip6_tnl_create2(dev);
+	if (!err && tb[IFLA_MTU])
+		ip6_tnl_change_mtu(dev, nla_get_u32(tb[IFLA_MTU]));
+
+	return err;
 }
 
 static int ip6_tnl_changelink(struct net_device *dev, struct nlattr *tb[],