proc: hide kernel addresses via %pK in /proc/<pid>/stack

This file is readable for the task owner.  Hide kernel addresses from
unprivileged users, leave them function names and offsets.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: Kees Cook <kees.cook@canonical.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>