fix streams_core/models/fields_m - offset calculation when using page numbers

since this fixes stuff in the fields model, other modules might be affected. Since use_page_numbers is hard coded in the pagination helper, this should actually be good for everything involved.

Only case, I can think of, where it will fail is if someone uses an offset without a limit. But on the other hand, that would'nt be a pagination so.... meh

But I've only checked the fields index in the streams admin.