Fix over-sized allocation of rtree leaf nodes.

Use the correct level metadata when allocating child nodes so that leaf
nodes don't end up over-sized (2^16 elements vs 2^4 elements).
diff --git a/src/rtree.c b/src/rtree.c
index d4a705a..0a42a98 100644
--- a/src/rtree.c
+++ b/src/rtree.c
@@ -171,7 +171,7 @@
     unsigned level)
 {
 
-	return (rtree_node_init(tsdn, rtree, level, &elm->child));
+	return (rtree_node_init(tsdn, rtree, level+1, &elm->child));
 }
 
 static int