Sometimes, when we parse some global info files, we can only care about
tasks which are taken into dump(such as file locks), which means their
pids are in the pstree.
So a function like this would be help.
Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>