mm/percpu.c: optimize the code in pcpu_setup_first_chunk() a little bit

This removes the need of local varibale 'chunk', and optimize the code
calling pcpu_alloc_first_chunk() to initialize reserved chunk and
dynamic chunk to make it simpler.

Signed-off-by: Baoquan He <bhe@redhat.com>
[Dennis: reworded first chunk init comment]
Signed-off-by: Dennis Zhou <dennis@kernel.org>