percpu: migrate percpu data structures to internal header

Migrates pcpu_chunk definition and a few percpu static variables to an
internal header file from mm/percpu.c. These will be used with debugfs
to expose statistics about percpu memory improving visibility regarding
allocations and fragmentation.

Signed-off-by: Dennis Zhou <dennisz@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>