virtio-balloon: free pbp more aggressively

virtio-balloon: free pbp more aggressively

Previous patches switched to a temporary pbp but that does not go far
enough: after device uses a buffer, guest is free to reuse it, so
tracking the page and freeing it later is wrong.

Free and reset the pbp after we push each element.

Fixes: ed48c59875b6 ("virtio-balloon: Safely handle BALLOON_PAGE_SIZE < host page size")
Cc: qemu-stable@nongnu.org #v4.0.0
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 1b47b37c33ec01ae1efc527f4c97f97f93723bc4)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>