writeback: clean up wb_dirty_limit()

The function name wb_dirty_limit(), its argument @dirty and the local
variable @wb_dirty are mortally confusing given that the function
calculates per-wb threshold value not dirty pages, especially given
that @dirty and @wb_dirty are used elsewhere for dirty pages.

Let's rename the function to wb_calc_thresh() and wb_dirty to
wb_thresh.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index ad5608d..9c8b7b5 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -49,7 +49,7 @@
 	struct bdi_writeback *wb = &bdi->wb;
 	unsigned long background_thresh;
 	unsigned long dirty_thresh;
-	unsigned long bdi_thresh;
+	unsigned long wb_thresh;
 	unsigned long nr_dirty, nr_io, nr_more_io, nr_dirty_time;
 	struct inode *inode;
 
@@ -67,7 +67,7 @@
 	spin_unlock(&wb->list_lock);
 
 	global_dirty_limits(&background_thresh, &dirty_thresh);
-	bdi_thresh = wb_dirty_limit(wb, dirty_thresh);
+	wb_thresh = wb_calc_thresh(wb, dirty_thresh);
 
 #define K(x) ((x) << (PAGE_SHIFT - 10))
 	seq_printf(m,
@@ -87,7 +87,7 @@
 		   "state:              %10lx\n",
 		   (unsigned long) K(wb_stat(wb, WB_WRITEBACK)),
 		   (unsigned long) K(wb_stat(wb, WB_RECLAIMABLE)),
-		   K(bdi_thresh),
+		   K(wb_thresh),
 		   K(dirty_thresh),
 		   K(background_thresh),
 		   (unsigned long) K(wb_stat(wb, WB_DIRTIED)),