rtmutex: Use waiter::task instead of current in remove_waiter()

rtmutex: Use waiter::task instead of current in remove_waiter()

remove_waiter() is used by the slowlock paths, but it is also used for
proxy-lock rollback in rt_mutex_start_proxy_lock() when invoked from
futex_requeue().

In the latter case waiter::task is not current, but remove_waiter()
operates on current for the dequeue operation. That results in several
problems:

  1) the rbtree dequeue happens without waiter::task::pi_lock being held

  2) the waiter task's pi_blocked_on state is not cleared, which leaves a
     dangling pointer primed for UAF around.

  3) rt_mutex_adjust_prio_chain() operates on the wrong top priority waiter
     task

Use waiter::task instead of current in all related operations in
remove_waiter() to cure those problems.

[ tglx: Fixup rt_mutex_adjust_prio_chain(), add a comment and amend the
  	changelog ]

Fixes: 8161239a8bcc ("rtmutex: Simplify PI algorithm and make highest prio task get lock")
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Keenan Dong <keenanat2000@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org