libdm: use delay_resume_if_extended

libdm: use delay_resume_if_extended

Update the logic towards more explicit logic.

Preload tree normally does not want to resume, only
in certain cases of extension or new loaded nodes can be
resumed. So introduce new internal variable delay_resume_if_extended
controlable by target.

Patch itself is not changing current existing behaviour,
and rather documents existing problem in more readable way.

lvm2 needs to introduce explicit mechanism how to support more
fain-grained (and safe) logic to i.e. resize thin-pool which
can be sitting on cached raid volume.