doc/RCU/listRCU: refine example code for eliminating stale data

This patch adjust the example code with following two purpose:

* reduce the confusion on not releasing e->lock
* emphasize e is valid and not stale with e->lock held

Signed-off-by: Wei Yang <[e-mail]>
CC: Boqun Feng <[e-mail]>
CC: Alan Huang <[e-mail]>
Reviewed-by: Alan Huang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Boqun Feng <[e-mail]>
Signed-off-by: Joel Fernandes <[e-mail]>

Documentation/RCU/listRCU.rst

···
 to accomplish this would be to add a ``deleted`` flag and a ``lock`` spinlock to the
 ``audit_entry`` structure, and modify audit_filter_task() as follows::
 
-	static enum audit_state audit_filter_task(struct task_struct *tsk)
+	static struct audit_entry *audit_filter_task(struct task_struct *tsk, char **key)
 	{
 		struct audit_entry *e;
 		enum audit_state   state;
···
 				if (e->deleted) {
 					spin_unlock(&e->lock);
 					rcu_read_unlock();
-					return AUDIT_BUILD_CONTEXT;
+					return NULL;
 				}
 				rcu_read_unlock();
 				if (state == AUDIT_STATE_RECORD)
 					*key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
-				return state;
+				/* As long as e->lock is held, e is valid and
+				 * its value is not stale */
+				return e;
 			}
 		}
 		rcu_read_unlock();
-		return AUDIT_BUILD_CONTEXT;
+		return NULL;
 	}
 
 The ``audit_del_rule()`` function would need to set the ``deleted`` flag under the