writeback: remove writeback_inodes_wbc

This was just an odd wrapper around writeback_inodes_wb.  Removing this
also allows to get rid of the bdi member of struct writeback_control
which was rather out of place there.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 660a87a..6e0b09a 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -340,14 +340,13 @@
 static void bdi_flush_io(struct backing_dev_info *bdi)
 {
 	struct writeback_control wbc = {
-		.bdi			= bdi,
 		.sync_mode		= WB_SYNC_NONE,
 		.older_than_this	= NULL,
 		.range_cyclic		= 1,
 		.nr_to_write		= 1024,
 	};
 
-	writeback_inodes_wbc(&wbc);
+	writeback_inodes_wb(&bdi->wb, &wbc);
 }
 
 /*