ext4: use a byte granularity cursor in ext4_mpage_readpages

Replace the next_block variable that is in units of file system blocks
and incorrectly uses the sector_t type with a byte offset, as that is
what both users want.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20260302141922.370070-15-hch@lst.de
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

Author: Christoph Hellwig

fs/ext4/readpage.c

@@ -215,11 +215,11 @@ static int ext4_mpage_readpages(struct inode *inode, struct fsverity_info *vi,
 	sector_t last_block_in_bio = 0;
 	const unsigned blkbits = inode->i_blkbits;
 	const unsigned blocksize = 1 << blkbits;
-	sector_t next_block;
 	sector_t block_in_file;
 	sector_t last_block;
 	sector_t last_block_in_file;
 	sector_t first_block;
+	loff_t pos;
 	unsigned page_block;
 	struct block_device *bdev = inode->i_sb->s_bdev;
 	int length;
@@ -249,7 +249,8 @@ static int ext4_mpage_readpages(struct inode *inode, struct fsverity_info *vi,
 
 		blocks_per_folio = folio_size(folio) >> blkbits;
 		first_hole = blocks_per_folio;
-		block_in_file = next_block = EXT4_PG_TO_LBLK(inode, folio->index);
+		pos = folio_pos(folio);
+		block_in_file = pos >> blkbits;
 		last_block = EXT4_PG_TO_LBLK(inode, folio->index + nr_pages);
 		last_block_in_file = (ext4_readpage_limit(inode) +
 				      blocksize - 1) >> blkbits;
@@ -342,8 +343,7 @@ static int ext4_mpage_readpages(struct inode *inode, struct fsverity_info *vi,
 		 * BIO off first?
 		 */
 		if (bio && (last_block_in_bio != first_block - 1 ||
-			    !fscrypt_mergeable_bio(bio, inode,
-				(loff_t)next_block << blkbits))) {
+			    !fscrypt_mergeable_bio(bio, inode, pos))) {
 		submit_and_realloc:
 			blk_crypto_submit_bio(bio);
 			bio = NULL;
@@ -355,8 +355,7 @@ static int ext4_mpage_readpages(struct inode *inode, struct fsverity_info *vi,
 			 */
 			bio = bio_alloc(bdev, bio_max_segs(nr_pages),
 					REQ_OP_READ, GFP_KERNEL);
-			fscrypt_set_bio_crypt_ctx(bio, inode,
-					(loff_t)next_block << blkbits, GFP_KERNEL);
+			fscrypt_set_bio_crypt_ctx(bio, inode, pos, GFP_KERNEL);
 			ext4_set_bio_post_read_ctx(bio, inode, vi);
 			bio->bi_iter.bi_sector = first_block << (blkbits - 9);
 			bio->bi_end_io = mpage_end_io;