[SCSI] scsi_debug: remove the duplicated code in resp_read and resp_write

resp_read and resp_write performs READ_* and WRITE_* commands
respectively. This sweeps up the similar code in them.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>