mm/percpu: Update the code comment when creating new chunk

The lock pcpu_alloc_mutex taking code has been moved to the beginning of
pcpu_allo() if it's non atomic allocation. So the code comment above
above pcpu_create_chunk() callsite need be updated.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>