So here it is. If root task dies on restore the roots yard
dir remains unrmdired :( Since we already know its name, we
can remove one from criu. By the time we get to this place
the sub mount namespace(s) are already dead and yard dir
is empty. But umounting should be done by tasks after
successfull restore, so keep depopulation there.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>