mm: thp: Introduce per-size thp sysfs interface

In preparation for adding support for anonymous small-sized THP,
introduce new sysfs structure that will be used to control the new
behaviours. A new directory is added under transparent_hugepage for each
supported THP size, and contains an `enabled` file, which can be set to
"global" (to inherrit the global setting), "always", "madvise" or
"never". For now, the kernel still only supports PMD-sized anonymous
THP, so only 1 directory is populated.

The first half of the change converts transhuge_vma_suitable() and
hugepage_vma_check() so that they take a bitfield of orders for which
the user wants to determine support, and the functions filter out all
the orders that can't be supported, given the current sysfs
configuration and the VMA dimensions. If there is only 1 order set in
the input then the output can continue to be treated like a boolean;
this is the case for most call sites.

The second half of the change implements the new sysfs interface. It has
been done so that each supported THP size has a `struct thpsize`, which
describes the relevant metadata and is itself a kobject. This is pretty
minimal for now, but should make it easy to add new per-thpsize files to
the interface if needed in future (e.g. per-size defrag). Rather than
keep the `enabled` state directly in the struct thpsize, I've elected to
directly encode it into huge_anon_orders_[always|madvise|global]
bitfields since this reduces the amount of work required in
transhuge_vma_suitable() which is called for every page fault.

The remainder is copied from Documentation/admin-guide/mm/transhuge.rst,
as modified by this commit. See that file for further details.

Transparent Hugepage Support for anonymous memory can be entirely
disabled (mostly for debugging purposes) or only enabled inside
MADV_HUGEPAGE regions (to avoid the risk of consuming more memory
resources) or enabled system wide. This can be achieved
per-supported-THP-size with one of::

	echo always >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
	echo madvise >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
	echo never >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled

where <size> is the hugepage size being addressed, the available sizes
for which vary by system. Alternatively it is possible to specify that
a given hugepage size will inherrit the global enabled setting::

	echo global >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled

The global (legacy) enabled setting can be set as follows::

	echo always >/sys/kernel/mm/transparent_hugepage/enabled
	echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
	echo never >/sys/kernel/mm/transparent_hugepage/enabled

By default, PMD-sized hugepages have enabled="global" and all other
hugepage sizes have enabled="never". If enabling multiple hugepage
sizes, the kernel will select the most appropriate enabled size for a
given allocation.

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