scsi_data_buffer structure (in preparation for bidi)

This patchset, which I submitted before, adds a new data structure, scsi_data_buffer, including everything for data transfer:

struct scsi_data_buffer {
	unsigned length;
	int resid;
	short sg_count;
	short __sg_count;
	struct scatterlist *sglist;
};

One scsi_data_buffer structure is embedded in struct scsi_cmnd for uni-directional transfer. All the members are just moved from scsi_cmnd structure to scsi_data_buffer structure. So nothing is added to scsi_cmnd structure.

After applying this patchset, llds must use the scsi data accessors. This patchset includes some conversation patches (most of them are patches that Boaz submitted before), however there is still some code that is needed to be converted.

Boaz, if you still have other conversation patches, please submit them.

I suspect that nobody cares about some of unconverted llds, which will be not compilable after this patchset. I'll fix ldds that people complain about.

It's difficult to test this patchset in -mm via scsi-misc. Jens, please send this to -mm via the block tree.