restore: fail restore if pgid or sid are not restored

Don't fail if a root non-init task has another sid, because
it's inherited from parent and can't be restored and
it's expected behaviour, when a subtree is dumped.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>