Btrfs: Fix balance panic

Mark the cloned backref_node as checked in clone_backref_node()
Signed-off-by: NYan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/relocation.c

@@ -1157,6 +1157,7 @@ static int clone_backref_node(struct btrfs_trans_handle *trans,
 	new_node->bytenr = dest->node->start;
 	new_node->level = node->level;
 	new_node->lowest = node->lowest;
+	new_node->checked = 1;
 	new_node->root = dest;
 
 	if (!node->lowest) {