net: Use ND_TYPE__EXTLINK for venet

In criu commit 96fdc01798ac60c44a8e40793d964fe918ddb83b
all external devices are to be assigned by EXTLINK
type. So we need to update our source code.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
task: Print restart block

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>