block: Use bdrv_set_backing_hd everywhere

We need to handle the coming backing_blocker properly, so don't open
code the assignment, instead, call bdrv_set_backing_hd to change
backing_hd.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -2652,13 +2652,11 @@ int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
     if (ret) {
         goto exit;
     }
-    new_top_bs->backing_hd = base_bs;
-
-    bdrv_refresh_limits(new_top_bs);
+    bdrv_set_backing_hd(new_top_bs, base_bs);
 
     QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry, next) {
         /* so that bdrv_close() does not recursively close the chain */
-        intermediate_state->bs->backing_hd = NULL;
+        bdrv_set_backing_hd(intermediate_state->bs, NULL);
         bdrv_unref(intermediate_state->bs);
     }
     ret = 0;

block/stream.c

@@ -60,7 +60,7 @@ static void close_unused_images(BlockDriverState *top, BlockDriverState *base,
     /* Must assign before bdrv_delete() to prevent traversing dangling pointer
      * while we delete backing image instances.
      */
-    top->backing_hd = base;
+    bdrv_set_backing_hd(top, base);
 
     while (intermediate) {
         BlockDriverState *unused;
@@ -72,7 +72,7 @@ static void close_unused_images(BlockDriverState *top, BlockDriverState *base,
 
         unused = intermediate;
         intermediate = intermediate->backing_hd;
-        unused->backing_hd = NULL;
+        bdrv_set_backing_hd(unused, NULL);
         bdrv_unref(unused);
     }
 

block/vvfat.c

@@ -2947,7 +2947,7 @@ static int enable_write_target(BDRVVVFATState *s)
     unlink(s->qcow_filename);
 #endif
 
-    s->bs->backing_hd = bdrv_new("", &error_abort);
+    bdrv_set_backing_hd(s->bs, bdrv_new("", &error_abort));
     s->bs->backing_hd->drv = &vvfat_write_target;
     s->bs->backing_hd->opaque = g_malloc(sizeof(void*));
     *(void**)s->bs->backing_hd->opaque = s;