percpu: explain why per_cpu_ptr_to_phys() is more complicated than necessary

Add comments about current per_cpu_ptr_to_phys implementation to
explain why the logic is more complicated than necessary.

-tj: relocated comment into kerneldoc comment

Signed-off-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>