bsg: add SCSI transport-level request support

This enables bsg to handle SCSI transport-level request like SAS
management protocol (SMP).

- add BSG_SUB_PROTOCOL_{SCSI_CMD, SCSI_TMF, SCSI_TRANSPORT} definitions.
- SCSI transport-level requests skip blk_verify_command().

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>