pid: Extend/Fix pid_vnr

pid_vnr returns the user space pid with respect to the pid namespace the
struct pid was allocated in.  What we want before we return a pid to user
space is the user space pid with respect to the pid namespace of current.

pid_vnr is a very nice optimization but because it isn't quite what we want
it is easy to use pid_vnr at times when we aren't certain the struct pid
was allocated in our pid namespace.

Currently this describes at least tiocgpgrp and tiocgsid in ttyio.c the
parent process reported in the core dumps and the parent process in
get_signal_to_deliver.

So unless the performance impact is huge having an interface that does what
we want instead of always what we want should be much more reliable and
much less error prone.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -1332,9 +1332,8 @@ struct pid_namespace;
  * from various namespaces
  *
  * task_xid_nr()     : global id, i.e. the id seen from the init namespace;
- * task_xid_vnr()    : virtual id, i.e. the id seen from the namespace the task
- *                     belongs to. this only makes sence when called in the
- *                     context of the task that belongs to the same namespace;
+ * task_xid_vnr()    : virtual id, i.e. the id seen from the pid namespace of
+ *                     current.
  * task_xid_nr_ns()  : id seen from the ns specified;
  *
  * set_task_vxid()   : assigns a virtual id to a task;