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>