ipvlan: fix IFLA_MTU ignored on NEWLINK

Commit 296d4856 ("ipvlan: inherit MTU from master device") adjusted
the mtu from the master device when creating a ipvlan device, but it
would also override the mtu value set in rtnl_create_link. It causes
IFLA_MTU param not to take effect.

So this patch is to not adjust the mtu if IFLA_MTU param is set when
creating a ipvlan device.

Fixes: 296d4856 ("ipvlan: inherit MTU from master device")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ipvlan/ipvlan_main.c

@@ -594,6 +594,7 @@ int ipvlan_link_new(struct net *src_net, struct net_device *dev,
 	ipvlan->phy_dev = phy_dev;
 	ipvlan->dev = dev;
 	ipvlan->sfeatures = IPVLAN_FEATURES;
+	if (!tb[IFLA_MTU])
 		ipvlan_adjust_mtu(ipvlan, phy_dev);
 	INIT_LIST_HEAD(&ipvlan->addrs);
 	spin_lock_init(&ipvlan->addrs_lock);