83ba7b07 cleans up the writeback.
So we don't use wb any more in get_next_work_item.
Let's remove unnecessary argument.

CC: Christoph Hellwig <hch@lst.de>
Signed-off-by: NMinchan Kim <minchan.kim@gmail.com>
Signed-off-by: NJens Axboe <jaxboe@fusionio.com>