staging: erofs: localize erofs_workgroup_get

Staticize erofs_workgroup_get since no external user
out of utils.c directly calls erofs_workgroup_get.

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