We don't need to clean_mnt_ns(), if we are goning to do pivot_root().

"""
pivot_root moves the root file system of the current process to the
directory put_old and makes new_root the new root file system.
"""

So I suggest to do pivot_root() and then detach the old root, all
other mount points will be unmounted automatically.

This patch fixes a problem, when a new root is mounted above a non-root
mount point. It's a default configuration for OpenVZ.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>