selftests/mm/cow: Generalize do_run_with_thp() helper

do_run_with_thp() prepares THP memory into different states before
running tests. We would like to reuse this logic to also test large anon
folios. So let's add a size parameter which tells the function what size
of memory it should operate on.

Remove references to THP and replace with LARGE, and fix up all existing
call sites to pass thpsize as the required size.

No functional change intended here, but a separate commit will add new
large anon folio tests that use this new capability.

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