lv: fix lock holder for external origin

lv: fix lock holder for external origin

External origin could be reloaded via more locks.
It's actually even more complex then thin-pool,
as it may be active on more nodes for linear LVs
(and maybe even more types).

External origin is always read-only thus unmodifiable
device so there should not be a problem accesing it
through multiple nodes.

Also for thin-pool check first presence of active thin-pool.

FIXME:
It's not easy to detect on which nodes this device is active
Thus manipulation with such device may require checking every
node and it active state and refresh.

But since such setup is quite complex to prepare and use,
hopefully there are not user trying to 'explore' this usage yet.