use lxc_putold as pivot_dir put dir, not mnt
use lxc_putold as pivot_dir put dir, not mnt

Using mnt means that lxc fstab entries do not work when placed under
the container's /mnt/ (i.e. /mnt/etc).

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

ostap/lxc,ostap/lxc