When a TASK_HELPER would exit just before a zombie, sometimes the signal
would get coalesced, and we would miss the zombie exit, causing us to block
forever waiting for the zombie to complete. Let's use an entirely different
strategy for waiting on zombies: explicitly wait on them with waitid, and
use WNOWAIT to prevent their data from actually being reaped.

v2: don't decrement nr_{tasks,threads} in the loop
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>