target: Generate failure for XCOPY I/O with non-zero scsi_status

This patch adds the missing non-zero se_cmd->scsi_status check required
for local XCOPY I/O within target_xcopy_issue_pt_cmd() to signal an
exception case failure.

This will trigger the generation of SAM_STAT_CHECK_CONDITION status
from within target_xcopy_do_work() process context code.

Reported-by: Thomas Glanzmann <thomas@glanzmann.de>
Reported-by: Douglas Gilbert <dgilbert@interlog.com>
Cc: Thomas Glanzmann <thomas@glanzmann.de>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>