[SCSI] scsi_debug: add bidi data transfer support

This enables fill_from_dev_buffer and fetch_to_dev_buffer to handle
bidi commands.

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>