sctp: fix addr_wq_timer race in sctp_free_addr_wq()

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.

Fixes: 4db67e808640 ("sctp: Make the address lists per network namespace")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/5dc95f295bdb5c3f60e880dd9aa5112dc5c071cc.1782757874.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>