[SCSI] implement scsi_data_buffer

In preparation for bidi we abstract all IO members of scsi_cmnd,
that will need to duplicate, into a substructure.

- Group all IO members of scsi_cmnd into a scsi_data_buffer
  structure.
- Adjust accessors to new members.
- scsi_{alloc,free}_sgtable receive a scsi_data_buffer instead of
  scsi_cmnd. And work on it.
- Adjust scsi_init_io() and  scsi_release_buffers() for above
  change.
- Fix other parts of scsi_lib/scsi.c to members migration. Use
  accessors where appropriate.

- fix Documentation about scsi_cmnd in scsi_host.h

- scsi_error.c
  * Changed needed members of struct scsi_eh_save.
  * Careful considerations in scsi_eh_prep/restore_cmnd.

- sd.c and sr.c
  * sd and sr would adjust IO size to align on device's block
    size so code needs to change once we move to scsi_data_buff
    implementation.
  * Convert code to use scsi_for_each_sg
  * Use data accessors where appropriate.

- tgt: convert libsrp to use scsi_data_buffer

- isd200: This driver still bangs on scsi_cmnd IO members,
  so need changing

[jejb: rebased on top of sg_table patches fixed up conflicts
and used the synergy to eliminate use_sg and sg_count]

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

include/scsi/scsi_eh.h

@@ -68,16 +68,14 @@ extern int scsi_get_sense_info_fld(const u8 * sense_buffer, int sb_len,
 extern int scsi_reset_provider(struct scsi_device *, int);
 
 struct scsi_eh_save {
+	/* saved state */
 	int result;
 	enum dma_data_direction data_direction;
 	unsigned char cmd_len;
 	unsigned char cmnd[MAX_COMMAND_SIZE];
+	struct scsi_data_buffer sdb;
 
-	void *buffer;
-	unsigned bufflen;
-	unsigned short use_sg;
-	int resid;
-
+	/* new command support */
 	struct scatterlist sense_sgl;
 };