datapath-windows: Release WFP allocated objects

datapath-windows: Release WFP allocated objects

Filter and sublayer WFP objects are created when the filter attach routine
is called and the tunnel filter is initialized, and they are released
in the uninitialize part. But the release part is done indirectly, through
the FwpmEngineClose function call which releases the latter objects when
the tunnel is uninitialized at filter detach.

We need to properly release the created objects and not rely that
FwpmEngineClose function does this for us, because if we decide to keep
the engine opened passed the filter detach those objects will continue to
exist.

Even though the FwpmEngineClose function releases the latter objects, we
still need this and the reason will became obvious in the next patch.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>