bcache: Convert btree_iter to struct btree_keys

More work to disentangle bset.c from struct btree

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

drivers/md/bcache/sysfs.c

@@ -460,7 +460,7 @@ lock_root:
 			rw_lock(false, b, b->level);
 		} while (b != c->root);
 
-		for_each_key_filter(b, k, &iter, bch_ptr_bad)
+		for_each_key_filter(&b->keys, k, &iter, bch_ptr_bad)
 			bytes += bkey_bytes(k);
 
 		rw_unlock(false, b);