dump: try to stop a task as soon as possible

We read /proc/pid/status to determine a task state, but if a task is
running in this moment, its state may be changed.

This patch stops tasks before reading their /proc/pid/status

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