locking/rtmutex: Fix ww_mutex deadlock check

Dan reported that rt_mutex_adjust_prio_chain() can be called with
.orig_waiter == NULL however commit a055fcc1 ("locking/rtmutex:
Return success on deadlock for ww_mutex waiters") unconditionally
dereferences it.

Since both call-sites that have .orig_waiter == NULL don't care for the
return value, simply disable the deadlock squash by adding the NULL
check.

Notably, both callers use the deadlock condition as a termination
condition for the iteration; once detected, we're sure (de)boosting is
done. Arguably [3] would be a more natural termination point, but I'm
not sure adding a third deadlock detection state would improve the code.

Fixes: a055fcc1

 ("locking/rtmutex: Return success on deadlock for ww_mutex waiters")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/YS9La56fHMiCCo75@hirez.programming.kicks-ass.net