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 small-sized THP, we would
like to reuse this logic to also test those smaller THP sizes. So let's
add a size parameter which tells the function what size THP it should
operate on.

A separate commit will utilize this change to add new tests for
small-sized THP, where available.

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