restore: create the root netns before running setup-namespaces scripts

runc restore executes criu with --emptyns network and set
a setup-namespaces script to restore a network namespace.

https://github.com/xemul/criu/issues/314



Looks-good-to: Pavel Emelyanov <xemul@virtuozzo.com>
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Fixes: 2189b9c71d3d ("net: allow to dump and restore more than one network namespace")
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>