writeback: remove writeback_control.more_io

When wbc.more_io was first introduced, it indicates whether there are
at least one superblock whose s_more_io contains more IO work. Now with
the per-bdi writeback, it can be replaced with a simple b_more_io test.

Acked-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index d0553f3..f43c479 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -560,12 +560,8 @@
 		iput(inode);
 		cond_resched();
 		spin_lock(&wb->list_lock);
-		if (wbc->nr_to_write <= 0) {
-			wbc->more_io = 1;
+		if (wbc->nr_to_write <= 0)
 			return 1;
-		}
-		if (!list_empty(&wb->b_more_io))
-			wbc->more_io = 1;
 	}
 	/* b_io is empty */
 	return 1;
@@ -708,7 +704,6 @@
 			wbc.older_than_this = &oldest_jif;
 		}
 
-		wbc.more_io = 0;
 		wbc.nr_to_write = write_chunk;
 		wbc.pages_skipped = 0;
 		wbc.inodes_written = 0;
@@ -740,7 +735,7 @@
 		/*
 		 * No more inodes for IO, bail
 		 */
-		if (!wbc.more_io)
+		if (list_empty(&wb->b_more_io))
 			break;
 		/*
 		 * Nothing written. Wait for some inode to