staging: erofs: rename strange variable names in z_erofs_vle_frontend

Previously, 2 members called `initial' and `cachedzone_la' are used
for applying caching policy (whether the workgroup is at either end),
which are hard to understand, rename them to `backmost' and `headoffset'.

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