use of shrinker in virtio balloon free page hinting

[Michael S. Tsirkin]

Wei, others,

ATM virtio_balloon_shrinker_scan will only get registered
when deflate on oom feature bit is set.

Not sure whether that's intentional.  Assuming it is:

virtio_balloon_shrinker_scan will try to locate and free
pages that are processed by host.
The above seems broken in several ways:
- count ignores the free page list completely
- if free pages are being reported, pages freed
  by shrinker will just get re-allocated again

I was unable to make this part of code behave in any reasonable
way - was shrinker usage tested? What's a good way to test that?

Thanks!

-- 
MST