block: Take graph rdlock in bdrv_drop_intermediate()

The function reads the parents list, so it needs to hold the graph lock.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-ID: <20230911094620.45040-18-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -5938,9 +5938,11 @@ int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
         backing_file_str = base->filename;
     }
 
+    bdrv_graph_rdlock_main_loop();
     QLIST_FOREACH(c, &top->parents, next_parent) {
         updated_children = g_slist_prepend(updated_children, c);
     }
+    bdrv_graph_rdunlock_main_loop();
 
     /*
      * It seems correct to pass detach_subchain=true here, but it triggers