kernel code reading: percpu allocate pcpu variable dynamicly

This reading I focus on how pcpu variable allcated and referenced dynamically.

Write a detailed operation in section 7.per_cpu_ptr() for the first chunk.

For other chunks, the base_addr is returned from the vm_areas, which I am not
familiar. Will leave this to the future reading.
kernel code reading: percpu chunk split and alloc
kernel code reading: free dynamicly allocate pcpu variable