kfence: await for allocation using wait_event

Patch series "kfence: optimize timer scheduling", v2.

We have observed that mostly-idle systems with KFENCE enabled wake up
otherwise idle CPUs, preventing such to enter a lower power state.
Debugging revealed that KFENCE spends too much active time in
toggle_allocation_gate().

While the first version of KFENCE was using all the right bits to be
scheduling optimal, and thus power efficient, by simply using wait_event()
+ wake_up(), that code was unfortunately removed.

As KFENCE was exposed to various different configs and tests, the
scheduling optimal code slowly disappeared.  First because of hung task
warnings, and finally because of deadlocks when an allocation is made by
timer code with debug objects enabled.  Clearly, the "fixes" were not too
friendly for devices that want to be power efficient.

Therefore, let's try a little harder to fix the hung task and deadlock
problems that we have with wait_event() + wake_up(), while remaining as
scheduling frie...