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>

include/linux/backing-dev.h

@@ -93,7 +93,7 @@ extern void wb_writeout_inc(struct bdi_writeback *wb);
 /*
  * 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;

mm/page-writeback.c

@@ -1545,7 +1545,7 @@ static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
 	 * actually dirty; with m+n sitting in the percpu
 	 * deltas.
 	 */
-	if (dtc->wb_thresh < 2 * wb_stat_error(wb)) {
+	if (dtc->wb_thresh < 2 * wb_stat_error()) {
 		wb_reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
 		dtc->wb_dirty = wb_reclaimable + wb_stat_sum(wb, WB_WRITEBACK);
 	} else {
@@ -1803,7 +1803,7 @@ pause:
 		 * more page. However wb_dirty has accounting errors.  So use
 		 * the larger and more IO friendly wb_stat_error.
 		 */
-		if (sdtc->wb_dirty <= wb_stat_error(wb))
+		if (sdtc->wb_dirty <= wb_stat_error())
 			break;
 
 		if (fatal_signal_pending(current))