mm: swap: Swap-out small-sized THP without splitting

The upcoming anonymous small-sized THP feature enables performance
improvements by allocating large folios for anonymous memory. However
I've observed that on an arm64 system running a parallel workload (e.g.
kernel compilation) across many cores, under high memory pressure, the
speed regresses. This is due to bottlenecking on the increased number of
TLBIs added due to all the extra folio splitting.

Therefore, solve this regression by adding support for swapping out
small-sized THP without needing to split the folio, just like is already
done for PMD-sized THP. This change only applies when CONFIG_THP_SWAP is
enabled, and when the swap backing store is a non-rotating block device
- these are the same constraints as for the existing PMD-sized THP
swap-out support.

Note that no attempt is made to swap-in THP here - this is still done
page-by-page, like for PMD-sized THP.

The main change here is to improve the swap entry allocator so that it
can allocate any power-of-2 number of contiguous entries between [4, (1
<< PMD_ORDER)]. This is done by allocating a cluster for each distinct
order and allocating sequentially from it until the cluster is full.
This ensures that we don't need to search the map and we get no
fragmentation due to alignment padding for different orders in the
cluster. If there is no current cluster for a given order, we attempt to
allocate a free cluster from the list. If there are no free clusters, we
fail the allocation and the caller falls back to splitting the folio and
allocates individual entries (as per existing PMD-sized THP fallback).

As far as I can tell, this should not cause any extra fragmentation
concerns, given how similar it is to the existing PMD-sized THP
allocation mechanism. There will be up to (PMD_ORDER-1) clusters in
concurrent use though. In practice, the number of orders in use will be
small though.

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