percpu: fix first chunk size calculation for populated bitmap

Variable populated, which is a member of struct pcpu_chunk, is used as a
unit of size of unsigned long.
However, size of populated is miscounted. So, I fix this minor part.

Fixes: 8ab16c43

 ("percpu: change the number of pages marked in the first_chunk pop bitmap")
Cc: <stable@vger.kernel.org> # 4.14+
Signed-off-by: Sunghyun Jin <mcsmonk@gmail.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>