Fix double spill.

This fixes an issue where split nodes can be double spilled. This is typically
not noticable but it can have large effects when bulk inserting as double
spilled nodes will get added to the freelist which will grow quickly.

node.go

@@ -9,9 +9,9 @@ import (
 // node represents an in-memory, deserialized page.
 type node struct {
 	bucket     *Bucket
-	dirty      bool
 	isLeaf     bool
 	unbalanced bool
+	spilled    bool
 	key        []byte
 	pgid       pgid
 	parent     *node
@@ -313,6 +313,9 @@ func (n *node) splitIndex(threshold int) (index, sz int) {
 // Returns an error if dirty pages cannot be allocated.
 func (n *node) spill() error {
 	var tx = n.bucket.tx
+	if n.spilled {
+		return nil
+	}
 
 	// Spill child nodes first. Child nodes can materialize sibling nodes in
 	// the case of split-merge so we cannot use a range loop. We have to check
@@ -346,6 +349,7 @@ func (n *node) spill() error {
 		_assert(p.id < tx.meta.pgid, "pgid (%d) above high water mark (%d)", p.id, tx.meta.pgid)
 		node.pgid = p.id
 		node.write(p)
+		node.spilled = true
 
 		// Insert into parent inodes.
 		if node.parent != nil {