datapath-windows: Avoid BSOD when cleaning up a tunnel vport

datapath-windows: Avoid BSOD when cleaning up a tunnel vport

If an error appears when creating a tunnel vport the cleanup is performed
twice, which causes the tunnel vport to be released also twice and
generate a BSOD.

This patch modifies the tunnel filter cleanup logic to avoid 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/97
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>