percpu: make pcpu_free_chunk() use pcpu_mem_free() instead of kfree()

commit 099a19d9('allow limited allocation before slab is online') made
pcpu_alloc_chunk() use pcpu_mem_zalloc() but forgot to update
pcpu_free_chunk() accordingly.  This doesn't cause any immediate
problema, but fix it for consistency.

tj: commit message updated

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>