force integers for appropriate values

non-nil per_page is always an integer, regardless of whether the
folio or page is ordinal or not.

current_page and explicitly assigned non-nil next_page on ordinal pages
are always integers; implicit non-nil next_page, previous_page, and
last_page are already calculated as integers.

Change-Id: Ifc9ee7f5d7c984abded07fa67fda3feea1222e1e