swap: turn get_swap_page() into folio_alloc_swap()

This removes an assumption that a large folio is HPAGE_PMD_NR pages
in size.

Link: https://lkml.kernel.org/r/20220504182857.4013401-8-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>