mm: convert swap_cluster_readahead and swap_vma_readahead to return a folio

shmem_swapin_cluster() immediately converts the page back to a folio, and
swapin_readahead() may as well call folio_file_page() once instead of
having each function call it.

[willy@infradead.org: avoid NULL pointer deref]
  Link: https://lkml.kernel.org/r/ZYI7OcVlM1voKfBl@casper.infradead.org
Link: https://lkml.kernel.org/r/20231213215842.671461-14-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>