mm: thp: Support allocation of anonymous small-sized THP

Introduce the logic to allow THP to be configured (through the new sysfs
interface we just added) to allocate large folios to back anonymous
memory, which are smaller than PMD-size. We call this new THP type
"small-sized THP".

These small-sized 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, but can be enabled at runtime
by writing to /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled
(see documentation in previous commit). The long term aim is to change
the default 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>