scsi: storvsc: Use scsi_cmd_to_rq() instead of scsi_cmnd.request

Prepare for removal of the request pointer by using scsi_cmd_to_rq()
instead. This patch does not change any functionality.

Link: https://lore.kernel.org/r/20210809230355.8186-1-bvanassche@acm.org


Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>