block/iscsi: cancel libiscsi task when ABORT TASK TMF completes

block/iscsi: cancel libiscsi task when ABORT TASK TMF completes

The libiscsi iscsi_task_mgmt_async() API documentation says:

  abort_task will also cancel the scsi task. The callback for the scsi
  task will be invoked with SCSI_STATUS_CANCELLED

The libiscsi implementation does not fulfil this promise.  The task's
callback is not invoked and its struct iscsi_pdu remains in the internal
list (effectively leaked).

This patch invokes the libiscsi iscsi_scsi_cancel_task() API to force
the task's callback to be invoked with SCSI_STATUS_CANCELLED when the
ABORT TASK TMF completes and the task's callback hasn't been invoked
yet.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20180215111526.2464-1-stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>