If the futex wait operation was interrupted by a signal or timed out, the
`FutexItem` must be dequeued and dropped. Otherwise, malicious user programs
could repeatedly issue futex wait operations to exhaust kernel memory.
Due to asynchronicity, this removal can't be done by queue position nor by
futex key match up:
* The position might have changed during the pause as some earlier futex might
have been dequeued
* If two futexes with the same key are enqueued and then one of them times out
or is interrupted, a removal by key would likely dequeue the wrong futex
Therefore, we need to perform a removal by unique global futex ID.
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