Fix rtmutex security issues

Cherry-pick "rtmutex: Use waiter::task instead of current in remove_waiter()" to fix CVE-2026-43499.

Cherry-pick "locking/rtmutex: Skip remove_waiter() when waiter is not enqueued" to avoid a regression in the first patch, which is also CVE-2026-5316.

This is completely untested so far.