kernel code test: fix prototype warning
kernel code test: nr_units
kernel code reading: free un-used percpu space

After allocation percpu space, it will try to free un-used spaces.
This can be divided into two category:
1. un-used units
   units per group is roundup by upa, so maybe un-used units.
2. un-used space in a unit
   unit_size is expanded, so there would be gap in each unit.
kernel code reading: pcpu value get