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

commit c7cdff0e ("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: c7cdff0e

 ("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>