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

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CVE-2026-43499

In the Linux kernel, the following vulnerability has been resolved: rtmutex: Use waiter::task instead of current in removewaiter removewaiter is used by the slowlock paths, but it is also used for proxy-lock rollback in rtmutexstartproxylock when invoked from futexrequeue. In the latter case...