filemap: Add folio_next_pos()

Replace the open-coded implementation in ocfs2 (which loses the top
32 bits on 32-bit architectures) with a helper in pagemap.h.

Fixes: 35edec1 (ocfs2: update truncate handling of partial clusters)
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20251024170822.1427218-2-willy@infradead.org
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: ocfs2-devel@lists.linux.dev
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Matthew Wilcox (Oracle)

fs/ocfs2/alloc.c

@@ -6892,7 +6892,7 @@ static void ocfs2_zero_cluster_folios(struct inode *inode, loff_t start,
 		ocfs2_map_and_dirty_folio(inode, handle, from, to, folio, 1,
 				&phys);
 
-		start = folio_next_index(folio) << PAGE_SHIFT;
+		start = folio_next_pos(folio);
 	}
 out:
 	if (folios)

include/linux/pagemap.h

@@ -941,6 +941,17 @@ static inline pgoff_t folio_next_index(const struct folio *folio)
 	return folio->index + folio_nr_pages(folio);
 }
 
+/**
+ * folio_next_pos - Get the file position of the next folio.
+ * @folio: The current folio.
+ *
+ * Return: The position of the folio which follows this folio in the file.
+ */
+static inline loff_t folio_next_pos(const struct folio *folio)
+{
+	return (loff_t)folio_next_index(folio) << PAGE_SHIFT;
+}
+
 /**
  * folio_file_page - The page for a particular index.
  * @folio: The folio which contains this index.