target: Change TCM_NON_EXISTENT_LUN response to ASC=LOGICAL UNIT NOT SUPPORTED

This patch changes transport_send_check_condition_and_sense() for
TCM_NON_EXISTENT_LUN emulation to use 0x25 (LOGICAL UNIT NOT SUPPORTED)
instead of the original 0x20 (INVALID COMMAND OPERATION CODE).  This is
helpful to distinguish between TCM_UNSUPPORTED_SCSI_OPCODE ASC=0x20
exceptions.

Signed-off-by: Nicholas A. Bellinger <nab@risingtidesystems.com>