Btrfs: Fix balance panic

Mark the cloned backref_node as checked in clone_backref_node()

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 1f5556a..0825e4e 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -1157,6 +1157,7 @@
 	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) {