mm: thp: Extend THP to allocate anonymous large folios

Introduce the logic to allow THP to be configured (through the new
anon_orders interface we just added) to allocate large folios to back
anonymous memory, which are smaller than PMD-size (for example order-2,
order-3, order-4, etc).

These THPs continue to be PTE-mapped, but in many cases can still
provide similar benefits to traditional PMD-sized THP: Page faults are
significantly reduced (by a factor of e.g. 4, 8, 16, etc. depending on
the configured order), but latency spikes are much less prominent
because the size of each page isn't as huge as the PMD-sized variant and
there is less memory to clear in each page fault. The number of per-page
operations (e.g. ref counting, rmap management, lru list management) are
also significantly reduced since those ops now become per-folio.

Some architectures also employ TLB compression mechanisms to squeeze
more entries in when a set of PTEs are virtually and physically
contiguous and approporiately aligned. In this case, TLB misses will
occur less often.

The new behaviour is disabled by default because the anon_orders
defaults to only enabling PMD-order, but can be enabled at runtime by
writing to anon_orders (see documentation in previous commit). The long
term aim is to default anon_orders to include suitable lower orders, but
there are some risks around internal fragmentation that need to be
better understood first.

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