locking/rtmutex: Dequeue waiter on ww_mutex deadlock

The rt_mutex based ww_mutex variant queues the new waiter first in the
lock's rbtree before evaluating the ww_mutex specific conditions which
might decide that the waiter should back out. This check and conditional
exit happens before the waiter is enqueued into the PI chain.

The failure handling at the call site assumes that the waiter, if it is the
top most waiter on the lock, is queued in the PI chain and then proceeds to
adjust the unmodified PI chain, which results in RB tree corruption.

Dequeue the waiter from the lock waiter list in the ww_mutex error exit
path to prevent this.

Fixes: add46132

 ("locking/rtmutex: Extend the rtmutex core to support ww_mutex")
Reported-by: Sebastian Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210825102454.042280541@linutronix.de