block: Restructure remove_file_or_backing_child()

As of a future patch, bdrv_replace_child_tran() will take a BdrvChild **
pointer.  Prepare for that by getting such a pointer and using it where
applicable, and (dereferenced) as a parameter for
bdrv_replace_child_tran().

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211111120829.81329-7-hreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211115145409.176785-7-kwolf@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

block.c

@@ -4887,30 +4887,33 @@ static void bdrv_remove_file_or_backing_child(BlockDriverState *bs,
                                               BdrvChild *child,
                                               Transaction *tran)
 {
+    BdrvChild **childp;
     BdrvRemoveFilterOrCowChild *s;
 
-    assert(child == bs->backing || child == bs->file);
-
     if (!child) {
         return;
     }
 
+    if (child == bs->backing) {
+        childp = &bs->backing;
+    } else if (child == bs->file) {
+        childp = &bs->file;
+    } else {
+        g_assert_not_reached();
+    }
+
     if (child->bs) {
-        bdrv_replace_child_tran(child, NULL, tran);
+        bdrv_replace_child_tran(*childp, NULL, tran);
     }
 
     s = g_new(BdrvRemoveFilterOrCowChild, 1);
     *s = (BdrvRemoveFilterOrCowChild) {
         .child = child,
-        .is_backing = (child == bs->backing),
+        .is_backing = (childp == &bs->backing),
     };
     tran_add(tran, &bdrv_remove_filter_or_cow_child_drv, s);
 
-    if (s->is_backing) {
-        bs->backing = NULL;
-    } else {
-        bs->file = NULL;
-    }
+    *childp = NULL;
 }
 
 /*