Currently each task subtracts number of zombies from
task_entries->nr_threads without locks, so if two tasks will do this
operation concurrently, the result may be unpredictable.

https://github.com/xemul/criu/issues/13

Cc: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>