Get rid of the possible/online CPUs mess

[krizhanovsky]

Motivation

The problem is addressed in #2023 (comment) : at the moment our code uses for_each_possible_cpu() in several places and even NR_CPUS, but it was never tested against (and even designed!) hot-plug or even switch on/off (e.g. with sysctl) CPUs. Some other places use for_each_online_cpus(). We have no idea whether Tempesta works with dynamically switched on/off or added/removed CPUs.

So in some cases we allocate extra memory for CPUs, which are never used. E.g. imagine how bad it is on basic Ubuntu 22:

$ grep 'CONFIG_NR_CPUS=\|CONFIG_HOTPLUG_CPU=' /boot/config-5.15.0-91-generic 
CONFIG_NR_CPUS=8192
CONFIG_HOTPLUG_CPU=y

Scope

Need to move to for_each_online_cpu() and remove NR_CPUS from fw/cache.c.

Testing

There is no need any specific testing, our regular CI is enough

Documentation

Probably it makes sense to update https://tempesta-tech.com/knowledge-base/Requirements/ if switching off CONFIG_HOTPLUG_CPU could provide any performance improvements due to simplified macros around the kernel.