erofs: fold in used-once helper erofs_workgroup_unfreeze_final()

It's expected that erofs_workgroup_unfreeze_final() won't
be used in other places. Let's fold it to simplify the code.

Link: https://lore.kernel.org/r/20200729180235.25443-1-hsiangkao@redhat.com


Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>