On restore all mount namespaces are restored in the root mntns and
sub-namecpeaces are restored in temorary places.

This function allows to get paths to these places.

It will be used in open_remap_ghost(), because it's called in the root
task, when other tasks are not forked yet.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>