mm: Tidy up shmem mTHP controls and stats

Previously we had a situation where shmem mTHP controls and stats were
not exposed for some supported sizes and were exposed for some
unsupported sizes. So let's clean that up.

Anon mTHP can support all large orders (2, PMD_ORDER). But shmem can
support all large orders (1, MAX_PAGECACHE_ORDER). However, per-size
shmem controls and stats were previously being exposed for all the anon
mTHP orders, meaning order-1 was not present, and for arm64 64K base
pages, orders 12 and 13 were exposed but were not supported internally.

Tidy this all up by defining ctrl and stats attribute groups for anon
and file separately. Anon ctrl and stats groups are populated for all
orders in THP_ORDERS_ALL_ANON and file ctrl and stats groups are
populated for all orders in THP_ORDERS_ALL_FILE_DEFAULT.

The side-effect of all this is that different hugepage-*kB directories
contain different sets of controls and stats, depending on which memory
types support that size. This approach is preferred over the
alternative, which is to populate dummy controls and stats for memory
types that do not support a given size.

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