afs: add bdi backing to mount session.

This ensures that dirty data gets flushed properly.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fs/afs/super.c b/fs/afs/super.c
index 14f6431..e932e5a 100644
--- a/fs/afs/super.c
+++ b/fs/afs/super.c
@@ -311,6 +311,7 @@
 	sb->s_magic		= AFS_FS_MAGIC;
 	sb->s_op		= &afs_super_ops;
 	sb->s_fs_info		= as;
+	sb->s_bdi		= &as->volume->bdi;
 
 	/* allocate the root inode and dentry */
 	fid.vid		= as->volume->vid;