lightnvm: pblk: make symbol write_buffer_size static

Fixes the following sparse warning:

drivers/lightnvm/pblk-init.c:23:14: warning:
 symbol 'write_buffer_size' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>