lock: add a new primitive cr_wait_until_greater (v2)

to wait while a futex value bigger than defined value.

v2: s/bigger/greater

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