writeback: make writeback_in_progress() take bdi_writeback instead of backing_dev_info

writeback_in_progress() currently takes @bdi and returns whether
writeback is in progress on its root wb (bdi_writeback).  In
preparation for cgroup writeback support, make it take wb instead.
While at it, make it an inline function.

This patch doesn't make any functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 682e3a6..e3b5c1d 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1455,7 +1455,7 @@
 			break;
 		}
 
-		if (unlikely(!writeback_in_progress(bdi)))
+		if (unlikely(!writeback_in_progress(wb)))
 			bdi_start_background_writeback(bdi);
 
 		if (!strictlimit)
@@ -1573,7 +1573,7 @@
 	if (!dirty_exceeded && wb->dirty_exceeded)
 		wb->dirty_exceeded = 0;
 
-	if (writeback_in_progress(bdi))
+	if (writeback_in_progress(wb))
 		return;
 
 	/*