sg: convert the non-data path to use the block layer

This patch converts the non data path to use the block layer functions
(blk_get_request, blk_execute_rq_nowait, etc) instead of uses
scsi_execute_async().

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Douglas Gilbert <dougg@torque.net>
Cc: Mike Christie <michaelc@cs.wisc.edu>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>