iomap: add a io_private field to struct iomap_ioend

Add a private data field to struct iomap_ioend so that the file system
can attach information to it.  Zoned XFS will use this for a pointer to
the open zone.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250206064035.2323428-9-hch@lst.de
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Christoph Hellwig

fs/iomap/ioend.c

@@ -23,6 +23,7 @@ struct iomap_ioend *iomap_init_ioend(struct inode *inode,
 	ioend->io_offset = file_offset;
 	ioend->io_size = bio->bi_iter.bi_size;
 	ioend->io_sector = bio->bi_iter.bi_sector;
+	ioend->io_private = NULL;
 	return ioend;
 }
 EXPORT_SYMBOL_GPL(iomap_init_ioend);

include/linux/iomap.h

@@ -370,6 +370,7 @@ struct iomap_ioend {
 	struct iomap_ioend	*io_parent;	/* parent for completions */
 	loff_t			io_offset;	/* offset in the file */
 	sector_t		io_sector;	/* start sector of ioend */
+	void			*io_private;	/* file system private data */
 	struct bio		io_bio;		/* MUST BE LAST! */
 };