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>