Use get_dep_chain() instead of directly using a blocks _dep_chain

Child blocks (whether nested or via includes) don't get a copy of the
dependency chain, so the above method should be used to ensure the block
looks at its parents dep chain.

Fixes #15996

lib/ansible/vars/__init__.py

@@ -232,7 +232,7 @@ class VariableManager:
             # sure it sees its defaults above any other roles, as we previously
             # (v1) made sure each task had a copy of its roles default vars
             if task and task._role is not None:
-                all_vars = combine_vars(all_vars, task._role.get_default_vars(dep_chain=task._block._dep_chain))
+                all_vars = combine_vars(all_vars, task._role.get_default_vars(dep_chain=task._block.get_dep_chain()))
 
         if host:
             # next, if a host is specified, we load any vars from group_vars
@@ -325,7 +325,7 @@ class VariableManager:
         if task:
             if task._role:
                 all_vars = combine_vars(all_vars, task._role.get_vars(include_params=False))
-                all_vars = combine_vars(all_vars, task._role.get_role_params(task._block._dep_chain))
+                all_vars = combine_vars(all_vars, task._role.get_role_params(task._block.get_dep_chain()))
             all_vars = combine_vars(all_vars, task.get_vars())
 
         if host: