[SCSI] convert sd to scsi_execute_req (and update the scsi_execute_req API)

This one removes struct scsi_request entirely from sd.  In the process,
I noticed we have no callers of scsi_wait_req who don't immediately
normalise the sense, so I updated the API to make it take a struct
scsi_sense_hdr instead of simply a big sense buffer.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>