fs, proc: introduce /proc/<pid>/task/<tid>/children entry

fs, proc: introduce /proc/<pid>/task/<tid>/children entry

When we do checkpoint of a task we need to know the list of children the
task, has but there is no easy and fast way to generate reverse
parent->children chain from arbitrary <pid> (while a parent pid is
provided in "PPid" field of /proc/<pid>/status).

So instead of walking over all pids in the system (creating one big
process tree in memory, just to figure out which children a task has) --
we add explicit /proc/<pid>/task/<tid>/children entry, because the kernel
already has this kind of information but it is not yet exported.

This is a first level children, not the whole process tree.

Signed-off-by: Cyrill Gorcunov <[email protected]>
Reviewed-by: Oleg Nesterov <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Serge Hallyn <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>