nilfs2: use device's backing_dev_info for btree node caches

Previously, default_backing_dev_info was used for the mapping of btree
node caches.  This uses device dependent backing_dev_info to allow
detailed control of the device for the btree node pages.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
diff --git a/fs/nilfs2/btnode.c b/fs/nilfs2/btnode.c
index 4cc07b2..0e01957 100644
--- a/fs/nilfs2/btnode.c
+++ b/fs/nilfs2/btnode.c
@@ -48,13 +48,14 @@
 
 static struct address_space_operations def_btnode_aops;
 
-void nilfs_btnode_cache_init(struct address_space *btnc)
+void nilfs_btnode_cache_init(struct address_space *btnc,
+			     struct backing_dev_info *bdi)
 {
 	btnc->host = NULL;  /* can safely set to host inode ? */
 	btnc->flags = 0;
 	mapping_set_gfp_mask(btnc, GFP_NOFS);
 	btnc->assoc_mapping = NULL;
-	btnc->backing_dev_info = &default_backing_dev_info;
+	btnc->backing_dev_info = bdi;
 	btnc->a_ops = &def_btnode_aops;
 }