sctp: fix addr_wq_timer race in sctp_free_addr_wq()

In the Linux kernel, the following vulnerability has been resolved: sctp: fix addr_wq_timer race in sctp_free_addr_wq() sctp_free_addr_wq() previously removed addr_wq_timer using timer_delete() while holding addr_wq_lock. However, timer_delete() does not guarantee that a currently running timer handler has completed. This allows a race with sctp_addr_wq_timeout_handler(), where the handler may still run after addr_waitq has been freed, acquire addr_wq_lock, and access freed memory, leading to a use-after-free. Fix this by calling timer_shutdown_sync() before taking addr_wq_lock. This guarantees that any in-flight timer handler has finished and prevents the timer from being re-armed during teardown, making subsequent cleanup safe.