hv_balloon: do adjust_managed_page_count() when ballooning/un-ballooning

Unlike virtio_balloon/virtio_mem/xen balloon drivers, Hyper-V balloon driver
does not adjust managed pages count when ballooning/un-ballooning and this leads
to incorrect stats being reported, e.g. unexpected 'free' output.

Note, the calculation in post_status() seems to remain correct: ballooned out
pages are never 'available' and we manually add dm->num_pages_ballooned to
'commited'.

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20201202161245.2406143-3-vkuznets@redhat.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>

drivers/hv/hv_balloon.c

@@ -1198,6 +1198,7 @@ static void free_balloon_pages(struct hv_dynmem_device *dm,
 		__ClearPageOffline(pg);
 		__free_page(pg);
 		dm->num_pages_ballooned--;
+		adjust_managed_page_count(pg, 1);
 	}
 }
 
@@ -1238,8 +1239,10 @@ static unsigned int alloc_balloon_pages(struct hv_dynmem_device *dm,
 			split_page(pg, get_order(alloc_unit << PAGE_SHIFT));
 
 		/* mark all pages offline */
-		for (j = 0; j < alloc_unit; j++)
+		for (j = 0; j < alloc_unit; j++) {
 			__SetPageOffline(pg + j);
+			adjust_managed_page_count(pg + j, -1);
+		}
 
 		bl_resp->range_count++;
 		bl_resp->range_array[i].finfo.start_page =