virtio_balloon_shrinker_scan should return number of system pages freed,
but because it's calling functions that deal with balloon pages, it gets
confused and sometimes returns the number of balloon pages.

It does not matter practically as the exact number isn't
used, but it seems better to be consistent in case someone
starts using this API.

Further, if we ever tried to iteratively leak pages as
virtio_balloon_shrinker_scan tries to do, we'd run into issues - this is
because freed_pages was accumulating total freed pages, but was also
subtracted on each iteration from pages_to_free, which can result in
either leaking less memory than we were supposed to free, or more if
pages_to_free underruns.

On a system with 4K pages we are lucky that we are never asked to leak
more than 128 pages while we can leak up to 256 at a time,
but it looks like a real issue for systems with page size != 4K.

Fixes: 71994620 ("virtio_balloon: replace oom notifier with shrinker")
Reported-by: Khazhismel Kumykov <khazhy@google.com>
Reviewed-by: Wei Wang <wei.w.wang@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>