[SCSI] scsi_debug: set resid to indicate no data-in when medium error

set resid to the requested data-in length when a MEDIUM ERROR is
simulated. This implies no valid data is returned in the data-in
buffer

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>