datapath-windows: Enable failure after restarting extension

datapath-windows: Enable failure after restarting extension

If the extension was previously enabled and running, after issuing a
restart, stop+start, the extension fails to be enabled. This happens
because the extension's DeviceObject is not yet initialized before the
FilterAttach routine is called.

This patch addresses this issue.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/96
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>