scsi: sd: Clean up sd_setup_read_write_cmnd()

Rework sd_setup_read_write_cmnd() so it becomes more readable. Put all the
sanity checking at the head of the function and sanitize the logged error
messages. Move the legacy SCSI logging calls to the end of the functions
and reduce conditional nesting.

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 ]
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>