SBC-3 (revision 35) says:

    The PARAMETER LIST LENGTH field specifies the length in bytes of the
    UNMAP parameter list that is available to be transferred from the
    Data-Out Buffer. If the parameter list length is greater than zero
    and less than 0008h (i.e., eight), then the device server shall
    terminate the command with CHECK CONDITION status with the sense key
    set to ILLEGAL REQUEST and the additional sense code set to
    PARAMETER LIST LENGTH ERROR. A PARAMETER LIST LENGTH set to zero
    specifies that no data shall be sent.

so our sense code for too-short descriptors was wrong, and we were
incorrectly failing commands that didn't transfer any descriptors.

While we're at it, also handle the UNMAP check:

    If the ANCHOR bit is set to one, and the ANC_SUP bit in the Logical
    Block Provisioning VPD page (see 6.6.4) is set to zero, then the
    device server shall terminate the command with CHECK CONDITION
    status with the sense key set to ILLEGAL REQUEST and the additional
    sense code set to INVALID FIELD IN CDB.

(chris boot: Fix wrong cut+paste comment in transport_send_check_condition_and_sense)
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>