futex: Check value after qemu_futex_wait()

futex: Check value after qemu_futex_wait()

futex(2) - Linux manual page
https://man7.org/linux/man-pages/man2/futex.2.html
> Note that a wake-up can also be caused by common futex usage patterns
> in unrelated code that happened to have previously used the futex
> word's memory location (e.g., typical futex-based implementations of
> Pthreads mutexes can cause this under some conditions).  Therefore,
> callers should always conservatively assume that a return value of 0
> can mean a spurious wake-up, and use the futex word's value (i.e.,
> the user-space synchronization scheme) to decide whether to continue
> to block or not.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Link: https://lore.kernel.org/r/20250529-event-v5-1-53b285203794@daynix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>