misc I/O submission cleanups

Hi Martin,

this series tidies up various loose ends in the SCSI I/O submission path.

Follow-Ups:
- Re: misc I/O submission cleanups
  - From: Qian Cai
- Re: misc I/O submission cleanups
  - From: Martin K. Petersen
- [PATCH 10/10] scsi: only start the request just before dispatching
  - From: Christoph Hellwig
- [PATCH 09/10] scsi: remove scsi_setup_cmnd and scsi_setup_fs_cmnd
  - From: Christoph Hellwig
- [PATCH 08/10] scsi: cleanup allocation and freeing of sgtables
  - From: Christoph Hellwig
- [PATCH 07/10] scsi: rename scsi_mq_prep_fn to scsi_prepare_cmd
  - From: Christoph Hellwig
- [PATCH 06/10] scsi: rename scsi_prep_state_check to scsi_device_state_check
  - From: Christoph Hellwig
- [PATCH 05/10] scsi: use rq_dma_dir in scsi_setup_cmnd
  - From: Christoph Hellwig
- [PATCH 04/10] scsi: simplify varlen CDB length checking
  - From: Christoph Hellwig
- [PATCH 03/10] scsi: move command size detection out of the fast path
  - From: Christoph Hellwig
- [PATCH 02/10] scsi: remove scsi_init_cmd_errh
  - From: Christoph Hellwig
- [PATCH 01/10] scsi: don't export scsi_device_from_queue
  - From: Christoph Hellwig