Fix JFFS2 sync silent failure

JFFS2 does not appear to set s_bdi anywhere.  And as of 32a88aa1,
__sync_filesystem() will return 0 if s_bdi is not set.  As a result,
sync_fs() is never called for jffs2 and whatever remains in the wbuf
will not make it to the device.

Fix that up by assigning the mtd bdi.

Signed-off-by: Jörn Engel <joern@logfs.org>
Acked-By: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/mtd/mtdsuper.c

@@ -13,6 +13,7 @@
 #include <linux/mtd/super.h>
 #include <linux/namei.h>
 #include <linux/ctype.h>
+#include <linux/slab.h>
 
 /*
  * compare superblocks to see if they're equivalent
@@ -44,6 +45,7 @@ static int get_sb_mtd_set(struct super_block *sb, void *_mtd)
 
 	sb->s_mtd = mtd;
 	sb->s_dev = MKDEV(MTD_BLOCK_MAJOR, mtd->index);
+	sb->s_bdi = mtd->backing_dev_info;
 	return 0;
 }