mm: Allocate large folios for anonymous memory

With all of the enabler patches in place, modify the anonymous memory
write allocation path so that it opportunistically attempts to allocate
a large folio up to `max_anon_folio_order()` size (This value is
ultimately configured by the architecture). This reduces the number of
page faults, reduces the size of (e.g. LRU) lists, and generally
improves performance by batching what were per-page operations into
per-(large)-folio operations.

If CONFIG_LARGE_ANON_FOLIO is not enabled (the default) then
`max_anon_folio_order()` always returns 0, meaning we get the existing
allocation behaviour.

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