target: Fix COMPARE_AND_WRITE ref leak for non GOOD status

This patch addresses a long standing bug where the commit phase
of COMPARE_AND_WRITE would result in a se_cmd->cmd_kref reference
leak if se_cmd->scsi_status returned non SAM_STAT_GOOD.

This would manifest first as a lost SCSI response, and eventual
hung task during fabric driver logout or re-login, as existing
shutdown logic waited for the COMPARE_AND_WRITE se_cmd->cmd_kref
to reach zero.

To address this bug, compare_and_write_post() has been changed
to drop the incorrect !cmd->scsi_status conditional that was
preventing *post_ret = 1 for being set during non SAM_STAT_GOOD
status.

This patch has been tested with SAM_STAT_CHECK_CONDITION status
from normal target_complete_cmd() callback path, as well as the
incoming __target_execute_cmd() submission failure path when
se_cmd->execute_cmd() returns non zero status.

Reported-by: Donald White <dew@datera.io>
Cc: Donald White <dew@datera.io>
Tested-by: Gary Guo <ghg@datera.io>
Cc: Gary Guo <ghg@datera.io>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: <stable@vger.kernel.org> # v3.12+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>