Handle pid namespaces in cgroups code

There's one place that works with task pids - its the "tasks" file in cgroups.
 The read/write handlers assume, that the pid values go to/come from the user
space and thus it is a virtual pid, i.e.  the pid as it is seen from inside a
namespace.

Tune the code accordingly.
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cgroup.c

@@ -1269,7 +1269,7 @@ static int attach_task_by_pid(struct cgroup *cgrp, char *pidbuf)
 
 	if (pid) {
 		rcu_read_lock();
-		tsk = find_task_by_pid(pid);
+		tsk = find_task_by_vpid(pid);
 		if (!tsk || tsk->flags & PF_EXITING) {
 			rcu_read_unlock();
 			return -ESRCH;
@@ -1955,7 +1955,7 @@ static int pid_array_load(pid_t *pidarray, int npids, struct cgroup *cgrp)
 	while ((tsk = cgroup_iter_next(cgrp, &it))) {
 		if (unlikely(n == npids))
 			break;
-		pidarray[n++] = task_pid_nr(tsk);
+		pidarray[n++] = task_pid_vnr(tsk);
 	}
 	cgroup_iter_end(cgrp, &it);
 	return n;