If we have tree threads, a following situation can occur:

T2: dec task_entries->nr_in_progress
T3: dec task_entries->nr_in_progress
T3: futex(&task_entries->nr_in_progress, WAKE_UP);
T1: unmap task_entries
T2: futex(&task_entries->nr_in_progress, WAKE_UP) -> EFAULT -> BUG_ON()

This patch adds a futex for synchronising threads.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>