bcachefs: bch2_seek_pagecache_data() folio conversion

This converts bch2_seek_pagecache_data() to handle large folios.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-io.c

@@ -3488,10 +3488,10 @@ loff_t bch2_remap_file_range(struct file *file_src, loff_t pos_src,
 static int folio_data_offset(struct folio *folio, unsigned offset)
 {
 	struct bch_folio *s = bch2_folio(folio);
-	unsigned i;
+	unsigned i, sectors = folio_sectors(folio);
 
 	if (s)
-		for (i = offset >> 9; i < PAGE_SECTORS; i++)
+		for (i = offset >> 9; i < sectors; i++)
 			if (s->s[i].state >= SECTOR_DIRTY)
 				return i << 9;
 
@@ -3519,12 +3519,10 @@ static loff_t bch2_seek_pagecache_data(struct inode *vinode,
 
 			folio_lock(folio);
 			offset = folio_data_offset(folio,
-					folio->index == start_index
-					? start_offset & (PAGE_SIZE - 1)
-					: 0);
+					max(folio_pos(folio), start_offset) -
+					folio_pos(folio));
 			if (offset >= 0) {
-				ret = clamp(((loff_t) folio->index << PAGE_SHIFT) +
-					    offset,
+				ret = clamp(folio_pos(folio) + offset,
 					    start_offset, end_offset);
 				folio_unlock(folio);
 				folio_batch_release(&fbatch);