datapath-windows: Fix BSOD caused by DV due to memory leaks.

If the OVS extension is enabled, Driver Verifier will issue a BSOD
due to memory leaks. This issue reproduces each time and the problem
is in the filter attach routine when the switch context is initialized.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/50
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>