md: fix rcu protection in md_wakeup_thread

In the Linux kernel, the following vulnerability has been resolved: md: fix rcu protection in md_wakeup_thread We attempted to use RCU to protect the pointer 'thread', but directly passed the value when calling md_wakeup_thread(). This means that the RCU pointer has been acquired before rcu_read_lock(), which renders rcu_read_lock() ineffective and could lead to a use-after-free.