IPoIB: Copy child MTU from parent

When creating a child interface, copy the MTU information from the
parent.  Otherwise when the child's multicast join completes, the MTU
will not be updated since the code does

	dev->mtu = min(priv->mcast_mtu, priv->admin_mtu);

and priv->admin_mtu will be set to 0.
Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/ulp/ipoib/ipoib_vlan.c

@@ -90,6 +90,9 @@ int ipoib_vlan_add(struct net_device *pdev, unsigned short pkey)
 	}
 
 	priv->max_ib_mtu = ppriv->max_ib_mtu;
+	/* MTU will be reset when mcast join happens */
+	priv->dev->mtu   = IPOIB_UD_MTU(priv->max_ib_mtu);
+	priv->mcast_mtu  = priv->admin_mtu = priv->dev->mtu;
 	set_bit(IPOIB_FLAG_SUBINTERFACE, &priv->flags);
 
 	priv->pkey = pkey;