bio: Fix bio_first_folio() for SPARSEMEM without VMEMMAP

It is possible for physically contiguous folios to have discontiguous
struct pages if SPARSEMEM is enabled and SPARSEMEM_VMEMMAP is not.
This is correctly handled by folio_page_idx(), so remove this open-coded
implementation.

Fixes: 640d193 (block: Add bio_for_each_folio_all())
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20250612144126.2849931-1-willy@infradead.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Matthew Wilcox (Oracle)

include/linux/bio.h

@@ -291,7 +291,7 @@ static inline void bio_first_folio(struct folio_iter *fi, struct bio *bio,
 
 	fi->folio = page_folio(bvec->bv_page);
 	fi->offset = bvec->bv_offset +
-			PAGE_SIZE * (bvec->bv_page - &fi->folio->page);
+			PAGE_SIZE * folio_page_idx(fi->folio, bvec->bv_page);
 	fi->_seg_count = bvec->bv_len;
 	fi->length = min(folio_size(fi->folio) - fi->offset, fi->_seg_count);
 	fi->_next = folio_next(fi->folio);