writeback: writeback_inodes_sb() should use bdi_start_writeback()

Pointless to iterate other devices looking for a super, when
we have a bdi mapping.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

fs/fs-writeback.c

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 52aa545..fb61178c 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1206,7 +1206,7 @@
 	nr_to_write = nr_dirty + nr_unstable +
 			(inodes_stat.nr_inodes - inodes_stat.nr_unused);
 
-	bdi_writeback_all(sb, nr_to_write);
+	bdi_start_writeback(sb->s_bdi, nr_to_write);
 }
 EXPORT_SYMBOL(writeback_inodes_sb);