restore: Bypass sync points of NS-related stages when not needed

When we don't restore any namespaces criu forces tasks to wake
it up two times simply to no-op and wake up tasks back. This
can be optimized by simply omitting the not needed wakeups.

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