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