datapath: Fix max MTU size on RHEL 7.5 kernel

Without the patch, in RHEL 7.5, the maximum configurable MTU of vport
internal device is 1500, which shall be 65535.  This patch fixes this
issue.

Fixes: 39ca338 ("datapath: compat: Fix build on RHEL 7.5")
Reported-by: Lucas Alvares Gomes <lucasagomes@gmail.com>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Tested-by: Greg Rose <gvrose8192@gmail.com>

datapath/vport-internal_dev.c

@@ -169,6 +169,8 @@ static void do_setup(struct net_device *netdev)
 
 #ifdef HAVE_NET_DEVICE_WITH_MAX_MTU
 	netdev->max_mtu = ETH_MAX_MTU;
+#elif HAVE_RHEL7_MAX_MTU
+	netdev->extended->max_mtu = ETH_MAX_MTU;
 #endif
 	netdev->netdev_ops = &internal_dev_netdev_ops;