mm: thp: Add "recommend" option for anon_orders

In addition to passing a bitfield of folio orders to enable for THP,
allow the string "recommend" to be written, which has the effect of
causing the system to enable the orders preferred by the architecture
and by the mm. The user can see what these orders are by subsequently
reading back the file.

Note that these recommended orders are expected to be static for a given
boot of the system, and so the keyword "auto" was deliberately not used,
as I want to reserve it for a possible future use where the "best" order
is chosen more dynamically at runtime.

Recommended orders are determined as follows:
  - PMD_ORDER: The traditional THP size
  - arch_wants_pte_order() if implemented by the arch
  - PAGE_ALLOC_COSTLY_ORDER: The largest order kept on per-cpu free list

arch_wants_pte_order() can be overridden by the architecture if desired.
Some architectures (e.g. arm64) can coalsece TLB entries if a contiguous
set of ptes map physically contigious, naturally aligned memory, so this
mechanism allows the architecture to optimize as required.

Here we add the default implementation of arch_wants_pte_order(), used
when the architecture does not define it, which returns -1, implying
that the HW has no preference.

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