This patch addresses a bug for backends such as IBLOCK that perform
asynchronous completion via transport_complete_cmd(), that will call
target_complete_failure_work() -> transport_generic_request_failure(),
upon exception status and invoke cmd->transport_complete_callback()
-> compare_and_write_callback() incorrectly during the failure case.

It adds a check for a non zero se_cmd->scsi_status within the first
invocation of compare_and_write_callback(), and will jump to out plus
up se_device->caw_sem before exiting the callback.
Reported-by: Thomas Glanzmann <thomas@glanzmann.de>
Tested-by: Thomas Glanzmann <thomas@glanzmann.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>