dump: Check for pids reuse at suspend

While we try to seize task it can die and give its pid to
somebody else. This can break pstree consistency. Check for
parent being valid after task is seized.

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