proc: Better ownership of files for non-dumpable tasks in user namespaces

Instead of making the files owned by the GLOBAL_ROOT_USER.  Make
non-dumpable files whose mm has always lived in a user namespace owned
by the user namespace root.  This allows the container root to have
things work as expected in a container.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>