virtio_balloon: fix increment of vb->num_pfns in fill_balloon()

commit c7cdff0 ("virtio_balloon: fix deadlock on OOM")
changed code to increment vb->num_pfns before call to
set_page_pfns(), which used to happen only after.

This patch fixes boot hang for me on ppc64le KVM guests.

Fixes: c7cdff0 ("virtio_balloon: fix deadlock on OOM")
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Wang <wei.w.wang@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_balloon.c

@@ -174,13 +174,12 @@ static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
 	while ((page = balloon_page_pop(&pages))) {
 		balloon_page_enqueue(&vb->vb_dev_info, page);
 
-		vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE;
-
 		set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
 		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
 		if (!virtio_has_feature(vb->vdev,
 					VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
 			adjust_managed_page_count(page, -1);
+		vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE;
 	}
 
 	num_allocated_pages = vb->num_pfns;