fix critical bugs and improve iSCSI protocol compliance

- Fix nil pointer dereference in BindISCSISession when existSess is nil
- Fix reversed logic in SPCLuOffline/SPCLuOnline (Online flag was swapped)
- Use negotiated MaxXmitDataSegmentLength for response PDU segmentation (issue #41)
- Fix debug log incorrectly using Warn level in SBCGetLbaStatus

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/port/iscsit/session.go

@@ -380,7 +380,7 @@ func (s *ISCSITargetDriver) BindISCSISession(conn *iscsiConnection) error {
 		return fmt.Errorf("initiator err, invalid request")
 	}
 
-	if existSess == nil && conn.loginParam.tsih != 0 &&
+	if existSess != nil && conn.loginParam.tsih != 0 &&
 		existSess.TSIH != conn.loginParam.tsih {
 		return fmt.Errorf("initiator err, no session")
 	}