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 <[email protected]>
Reviewed-by: Nikolay Borisov <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>
9 files changed