iomap: add per-iomap_iter private data

Allow the file system to keep state for all iterations.  For now only
wire it up for direct I/O as there is an immediate need for it there.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>