Re: [PATCH RESEND 2/2] pidns: Expose task pid_ns_for_children to userspace

pid_ns_for_children set by a task is known only to the task itself,
and it's impossible to identify it from outside.

It's a big problem for checkpoint/restore software like CRIU,
because it can't correctly handle tasks, that do setns(CLONE_NEWPID)
in proccess of their work.

This patch solves the problem, and it exposes pid_ns_for_children
to ns directory in standard way with the name "pid_for_children":

~# ls /proc/5531/ns -l | grep pid
lrwxrwxrwx 1 root root 0 Jan 14 16:38 pid -> pid:[4026531836]
lrwxrwxrwx 1 root root 0 Jan 14 16:38 pid_for_children -> pid:[4026532286]

--- a/fs/proc/namespaces.c
+++ b/fs/proc/namespaces.c

@@ -23,6 +23,7 @@ static const struct proc_ns_operations *ns_entries[] = {
 #endif
 #ifdef CONFIG_PID_NS
 	&pidns_operations,
+	&pidns_for_children_operations,
 #endif