virtio_balloon: prevent pfn array overflow

Make sure, at build time, that pfn array is big enough to hold a single
page.  It happens to be true since the PAGE_SHIFT value at the moment is
20, which is 1M - exactly 256 4K balloon pages.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>

drivers/virtio/virtio_balloon.c

@@ -158,6 +158,8 @@ static void set_page_pfns(struct virtio_balloon *vb,
 {
 	unsigned int i;
 
+	BUILD_BUG_ON(VIRTIO_BALLOON_PAGES_PER_PAGE > VIRTIO_BALLOON_ARRAY_PFNS_MAX);
+
 	/*
 	 * Set balloon pfns pointing at this page.
 	 * Note that the first pfn points at start of the page.