Btrfs: Add checking of csum tree in balancing code

This updates the space balancing code for the
new checksum format.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index c72f4f3..6f042de 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1097,6 +1097,8 @@
 		return fs_info->chunk_root;
 	if (location->objectid == BTRFS_DEV_TREE_OBJECTID)
 		return fs_info->dev_root;
+	if (location->objectid == BTRFS_CSUM_TREE_OBJECTID)
+		return fs_info->csum_root;
 
 	root = radix_tree_lookup(&fs_info->fs_roots_radix,
 				 (unsigned long)location->objectid);