selftests/mm: Factor out thp settings management

The khugepaged test has a useful framework for save/restore/pop/push of
all thp settings via the sysfs interface. This will be useful to
explicitly control small-sized THP settings in other tests, so let's
move it out of khugepaged and into its own thp_settings.[c|h] utility.

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