fix: backward compatible building with kernel <5.3

[gnought]

This is a further fix on 122662f5c19864e3340cc44e703ae24b75a26ca7 __smp_processor_id only defines since kernel version 5.3+. It could be easily looked up into https://elixir.bootlin.com/linux/v5.3/source/include/linux/smp.h#L212