virtio-balloon: Simplify deflate with pbp

virtio-balloon: Simplify deflate with pbp

Let's simplify this - the case we are optimizing for is very hard to
trigger and not worth the effort. If we're switching from inflation to
deflation, let's reset the pbp.

Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190722134108.22151-4-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 2ffc49eea1bbd454913a88a0ad872c2649b36950)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>