Set descriptors to restart a current syscall

If a task is sleep in syscall, it should be restared.
This logic is moved from kernel do_signal().

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