scsi: sd: Create helper functions for read/write commands

Create a helper function for each of the 6, 10, 16 and 32-byte READ/WRITE
variants and use those when setting up reads and writes.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
[ bvanassche: ported this patch from kernel v4.11 to kernel v5.0 and made
  function names shorter. ]
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>