mm/huge_memory: work on folio->swap instead of page->private when splitting folio

Let's work on folio->swap instead.  While at it, use folio_test_anon() and
folio_test_swapcache() -- the original folio remains valid even after
splitting (but is then an order-0 folio).

We can probably convert a lot more to folios in that code, let's focus on
folio->swap handling only for now.

Link: https://lkml.kernel.org/r/20230821160849.531668-5-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Chris Li <chrisl@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>