restore: Rename CR_STATE_RESTORE_NS -> _ROOT_TASK

The stage name is what tasks do, not what criu waits for.
When the first stage is started we want the root task to
come up, rather than namespaces to get restored.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>