hammer_btree_lock_children
int hammer_btree_lock_children(hammer_cursor_t cursor, int depth,
error = hammer_btree_lock_children(cursor, 1, &lockroot, NULL);
error = hammer_btree_lock_children(
error = hammer_btree_lock_children(cursor, 2, &lockroot, lcache);
error = hammer_btree_lock_children(cursor, 1, &lockroot, NULL);