mm: swap: Scan for free swap entries in allocated clusters

Previously mTHP would only be swapped out if a CPU could allocate itself
a free cluster from which to allocate mTHP-sized contiguous swap entry
blocks. But for a system making heavy use of swap, after a while
fragmentation ensures there are no available free clusters and therefore
the swap entry allocation fails and forces the mTHP to be split to base
pages which then get swap entries allocated by scanning the swap file
for free individual pages.

But when swap entries are freed, this makes holes in the clusters, and
often it would be possible to allocate new mTHP swap entries in those
holes.

So if we fail to allocate a free cluster, scan through the clusters
until we find one that is in use and contains swap entries of the order
we require. Then scan it until we find a suitably sized and aligned
hole. We keep a per-order "next cluster to scan" pointer so that future
scanning can be picked up from where we last left off. And if we scan
through all clusters without finding a suitable hole, we give up to
prevent live lock.

Running the test case provided by Barry Song at the below link, I can
see swpout fallback rate, which was previously 100% after a few
iterations, falls to 0% and stays there for all 100 iterations. This is
also the case when sprinkling in some non-mTHP allocations ("-s") too.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/linux-mm/20240615084714.37499-1-21cnbao@gmail.com/