staging: erofs: fold in `__update_workgrp_llen'

There is the only one user to use `__update_workgrp_llen'.
Fold it in `z_erofs_vle_work_iter_begin' and cleanup related code.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>