selftests/mm: Support small-sized THP interface in thp_settings

Save and restore the new per-size hugepage enabled setting, if available
on the running kernel.

Since the number of per-size directories is not fixed, solve this as
simply as possible by catering for a maximum number in the thp_settings
struct (20). Each array index is the order. The value of THP_NEVER is
changed to 0 so that all of these new settings default to THP_NEVER and
the user only needs to fill in the ones they want to enable.

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