writeback: remove unused function parameter

The parameter `struct bdi_writeback *wb` is not been used in the
function body.  Remove it.

Link: http://lkml.kernel.org/r/1509685485-15278-1-git-send-email-wanglong19@meituan.com
Signed-off-by: Wang Long <wanglong19@meituan.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 7360e79..e54e7e0 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -93,7 +93,7 @@
 /*
  * maximal error of a stat counter.
  */
-static inline unsigned long wb_stat_error(struct bdi_writeback *wb)
+static inline unsigned long wb_stat_error(void)
 {
 #ifdef CONFIG_SMP
 	return nr_cpu_ids * WB_STAT_BATCH;