Two folio fixes for 5.18:

 - Fix a race when we were calling folio_next() in the BIO folio iter
   without holding a reference, meaning the folio could be split or freed,
   and we'd jump to the next page instead of the intended next folio.

 - Fix readahead creating single-page folios instead of the intended
   large folios when doing reads that are not a power of two in size.
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