fs-writeback: fix kernel-doc warnings

Fix kernel-doc to match the function's changed args.

Warning(fs/fs-writeback.c:190): No description found for parameter 'args'
Warning(fs/fs-writeback.c:190): Excess function parameter 'sb' description in 'bdi_queue_work_onstack'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

fs/fs-writeback.c

@@ -179,7 +179,7 @@ static void bdi_alloc_queue_work(struct backing_dev_info *bdi,
 
 /**
  * bdi_queue_work_onstack - start and wait for writeback
- * @sb: write inodes from this super_block
+ * @args: parameters to control the work queue writeback
  *
  * Description:
  *   This function initiates writeback and waits for the operation to