GC_MERGE_NODES
struct btree *new_nodes[GC_MERGE_NODES];
while (nodes < GC_MERGE_NODES && !IS_ERR_OR_NULL(r[nodes].b))
struct gc_merge_info r[GC_MERGE_NODES];
memmove(r + 1, r, sizeof(r[0]) * (GC_MERGE_NODES - 1));