virtio-balloon: don't track subpages for the PBP

virtio-balloon: don't track subpages for the PBP

As ramblocks cannot get removed/readded while we are processing a bulk
of inflation requests, there is no more need to track the page size
in form of the number of subpages.

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190725113638.4702-8-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 9a7ca8a7c920360db9dcaf616ca6f1440c025043)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>