btrfs_read_lock_root_node
b = btrfs_read_lock_root_node(root);
cur = btrfs_read_lock_root_node(root);
struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root);
eb = btrfs_read_lock_root_node(extent_root);
eb_root = btrfs_read_lock_root_node(root);