selftests/mm/khugepaged: Enlighten for multi-size THP

The `collapse_max_ptes_none` test was previously failing when a THP size
less than PMD-size had enabled="always". The root cause is because the
test faults in 1 page less than the threshold it set for collapsing. But
when THP is enabled always, we "over allocate" and therefore the
threshold is passed, and collapse unexpectedly succeeds.

Solve this by enlightening khugepaged selftest. Add a command line
option to pass in the desired THP size that should be used for all
anonymous allocations. The harness will then explicitly configure a THP
size as requested and modify the `collapse_max_ptes_none` test so that
it faults in the threshold minus the number of pages in the configured
THP size. If no command line option is provided, default to order 0, as
per previous behaviour.

I chose to use an order in the command line interface, since this makes
the interface agnostic of base page size, making it easier to invoke
from run_vmtests.sh.

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