mnt: Don't delay external mount points

It looks like criu constantly postpones external bind mounts. I'm trying to resolve
when we manage to break this (when I did ext-mount-map they for some reason didn't).
Meanwhile, this patch fixes it back.

Reported-by: Saied Kazemi <saied@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>