Add memory barrier

To avoid queuing an incomplete flush job
we have to insert memory barrier.
Adding write barrier before migrate_linked_segments

I don't know if this barrier needs actually
Merge branch 'feature/mikulas-memory-barrier' into develop
FIX: add wake_up(wq) after setting blockup via message

- without this, daemons will be sleeping forever
  after blockup set to 0
- plus, code clean up

XFS stalls when blockup set to 1 manually.
What is happening is still in the dark