staging: erofs: redefine where `owned_workgrp_t' points

By design, workgroups are queued in the form of linked lists.

Previously, it points to the next `z_erofs_vle_workgroup',
which isn't flexible enough to simplify `z_erofs_vle_submit_all'.

Let's fix it by pointing to the next `owned_workgrp_t' and use
container_of to get its coresponding `z_erofs_vle_workgroup'.

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