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>