If we fail to create temporary directory for doing a clean mount we can
make mount clean reusing the code which enters new mountns to umount
overmounts. As when last process exits mntns all mounts are implicitly
cleaned from children, see in kernel source - sys_exit->do_exit
->exit_task_namespaces->switch_task_namespaces->free_nsproxy
->put_mnt_ns->umount_tree->drop_collected_mounts->umount_tree:

    /* Hide the mounts from mnt_mounts */
    list_for_each_entry(p, &tmp_list, mnt_list) {
            list_del_init(&p->mnt_child);
    }

Fixes commit b6cfb1ce2948 ("mount: make open_mountpoint handle overmouts
properly")

https://github.com/checkpoint-restore/criu/issues/520Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Acked-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>