selftests/mm/cow: Generalize do_run_with_thp() helper

do_run_with_thp() prepares (PMD-sized) THP memory into different states
before running tests. With the introduction of THP orders that are
smaller than PMD_ORDER, we would like to reuse this logic to also test
those smaller orders. So let's add a size parameter which tells the
function what size THP it should operate on.

No functional change intended here, but a separate commit will add new
tests for smaller order THP, where available.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>