virtio-balloon: Revert "virtio-balloon: Switch back to OOM handler for...

virtio-balloon: Revert "virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM"

This reverts commit 5a6b4cc5.

It has been queued properly in the akpm tree, this version is just
creating conflicts.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>