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: NDarrick J. Wong <djwong@kernel.org>
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>