crush: fix tree node weight lookup

Fix the node weight lookup for tree buckets by using a correct accessor.

Reflects ceph.git commit d287ade5bcbdca82a3aef145b92924cf1e856733.

Reviewed-by: Alex Elder <elder@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>
diff --git a/include/linux/crush/crush.h b/include/linux/crush/crush.h
index 158a4d2..7c47508 100644
--- a/include/linux/crush/crush.h
+++ b/include/linux/crush/crush.h
@@ -166,4 +166,9 @@
 extern void crush_destroy_bucket(struct crush_bucket *b);
 extern void crush_destroy(struct crush_map *map);
 
+static inline int crush_calc_tree_node(int i)
+{
+	return ((i+1) << 1)-1;
+}
+
 #endif