To this end, fix races when updating 'acpi_tasks_hiwater'.
In particular, one thread could enqueue first at a lower but
still-greater-than-the-current-maximum index, another thread then
enqueuing at some higher index, and then the first thread would update
'acpi_tasks_hiwater' while the second was exactly after the 'if (i >
acpi_tasks_hiwater)' but just before attempting the atomic_cmpset_int()
that updates 'acpi_task_hiwater', which then would fail (and the return
value was ignored), leading to the transient maximum not being recorded.