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/scsi/sbc.go

@@ -789,7 +789,7 @@ func SBCGetLbaStatus(host int, cmd *api.SCSICommand) api.SAMStat {
 	tl = getSCSIReadWriteCount(scb)
 	// Verify that we are not doing i/o beyond the end-of-lun
 	totalBlocks = dev.Size >> dev.BlockShift
-	log.Warnf("DEBUG: dev.Size=%d, BlockShift=%d, totalBlocks=%d", dev.Size, dev.BlockShift, totalBlocks)
+	log.Debugf("dev.Size=%d, BlockShift=%d, totalBlocks=%d", dev.Size, dev.BlockShift, totalBlocks)
 	if lba >= totalBlocks || lba+uint64(tl) < lba || lba+uint64(tl) > totalBlocks {
 		key = ILLEGAL_REQUEST
 		asc = ASC_LBA_OUT_OF_RANGE

pkg/scsi/spc.go

@@ -38,7 +38,7 @@ func SPCIllegalOp(host int, cmd *api.SCSICommand) api.SAMStat {
 }
 
 func SPCLuOffline(lu *api.SCSILu) error {
-	lu.Attrs.Online = true
+	lu.Attrs.Online = false
 	return nil
 }
 
@@ -47,7 +47,7 @@ func SPCLuOnline(lu *api.SCSILu) error {
 		return fmt.Errorf("lu prevent removal")
 	}
 
-	lu.Attrs.Online = false
+	lu.Attrs.Online = true
 	return nil
 }