pstree: add a function to check if a task is in pstree

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>