IB/ipoib: Set MTU to max allowed by mode when mode changes

When switching between modes (datagram / connected) change the MTU
accordingly.
datagram mode up to 4K, connected mode up to (64K - 0x10).
Signed-off-by: NELi Cohen <eli@mellanox.com>
Signed-off-by: NErez Shitrit <erezsh@mellanox.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>

drivers/infiniband/ulp/ipoib/ipoib_main.c

@@ -232,6 +232,7 @@ int ipoib_set_mode(struct net_device *dev, const char *buf)
 		ipoib_warn(priv, "enabling connected mode "
 			   "will cause multicast packet drops\n");
 		netdev_update_features(dev);
+		dev_set_mtu(dev, ipoib_cm_max_mtu(dev));
 		rtnl_unlock();
 		priv->tx_wr.send_flags &= ~IB_SEND_IP_CSUM;