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: ELi Cohen <[e-mail]>
Signed-off-by: Erez Shitrit <[e-mail]>
Signed-off-by: Doug Ledford <[e-mail]>

drivers/infiniband/ulp/ipoib/ipoib_main.c

···
 		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;