procfs: Print task uids and gids in the userns that opened the proc file

Instead of using current_userns() use the userns of the opener
of the file so that if the file is passed between processes
the contents of the file do not change.

Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>