When looking for a dirty bitmap to share, we should handle filters by
just including them in the search (so they do not break backing chains).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 nbd/server.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index fbd51b48a7..a1c73ce957 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1511,13 +1511,13 @@ NBDExport *nbd_export_new(BlockDriverState *bs, uint64_t dev_offset,
     if (bitmap) {
         BdrvDirtyBitmap *bm = NULL;
 
-        while (true) {
+        while (bs) {
             bm = bdrv_find_dirty_bitmap(bs, bitmap);
-            if (bm != NULL || bs->backing == NULL) {
+            if (bm != NULL) {
                 break;
             }
 
-            bs = bs->backing->bs;
+            bs = bdrv_filtered_bs(bs);
         }
 
         if (bm == NULL) {
-- 
2.21.0