mnt: Fix slave mounts order estimation in can_mount_now

For slave mount we need to make sure that all childrent
mountpoints of the master peer is mounted before we can
proceed otherwise there gonna be duplicated mountpoints
after the restore.

https://jira.sw.ru/browse/PSBM-39957



Signed-off-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>