target: simplify code around transport_get_sense_data

The error conditions in transport_get_sense_data are superfluous
and complicate the code unnecessarily:

* SCF_TRANSPORT_TASK_SENSE is checked in the caller;

* it's simply part of the invariants of dev->transport->get_sense_buffer
  that it must be there if transport_complete ever returns 1, and that
  it must not return NULL.  Besides, the entire callback will disappear
  with the next patch.

* similarly in the caller we can expect that sense data is only sent
  for non-zero cmd->scsi_status.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>