fix: handle pipelined SCSI commands during Data-Out collection

The Linux kernel iSCSI initiator pipelines multiple SCSI commands on
the same TCP connection (command queuing). When a write needs R2T for
data beyond the immediate portion, collectDataOut may read a pipelined
SCSI command instead of the expected Data-Out PDU.

Fix: queue non-Data-Out PDUs received during collectDataOut into a
pending buffer. The main dispatch loop drains pending PDUs before
reading from the connection. This correctly handles interleaved
commands during multi-PDU write transfers.

Bug found during WSL2 smoke test: mkfs.ext4 hangs at "Writing
superblocks" because inode table zeroing sends large writes that
exceed FirstBurstLength, triggering R2T while the kernel has already
queued the next command.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

weed/storage/blockvol/iscsi/session.go

@@ -48,6 +48,11 @@ type Session struct {
 	// Data sequencing
 	dataInWriter *DataInWriter
 
+	// PDU queue for commands received during Data-Out collection.
+	// The initiator pipelines commands; we may read a SCSI Command
+	// while waiting for Data-Out PDUs.
+	pending []*PDU
+
 	// Shutdown
 	closed   atomic.Bool
 	closeErr error
@@ -80,7 +85,7 @@ func (s *Session) HandleConnection() error {
 	defer s.close()
 
 	for !s.closed.Load() {
-		pdu, err := ReadPDU(s.conn)
+		pdu, err := s.nextPDU()
 		if err != nil {
 			if s.closed.Load() {
 				return nil
@@ -101,6 +106,17 @@ func (s *Session) HandleConnection() error {
 	return nil
 }
 
+// nextPDU returns the next PDU to process, draining the pending queue
+// (populated during Data-Out collection) before reading from the connection.
+func (s *Session) nextPDU() (*PDU, error) {
+	if len(s.pending) > 0 {
+		pdu := s.pending[0]
+		s.pending = s.pending[1:]
+		return pdu, nil
+	}
+	return ReadPDU(s.conn)
+}
+
 // Close terminates the session.
 func (s *Session) Close() error {
 	s.closed.Store(true)
@@ -274,14 +290,17 @@ func (s *Session) collectDataOut(collector *DataOutCollector, itt uint32) error
 		}
 		r2tSN++
 
-		// Read Data-Out PDUs until F-bit
+		// Read Data-Out PDUs until F-bit.
+		// The initiator may pipeline other commands; queue them for later.
 		for {
 			doPDU, err := ReadPDU(s.conn)
 			if err != nil {
 				return err
 			}
 			if doPDU.Opcode() != OpSCSIDataOut {
-				return fmt.Errorf("expected Data-Out, got %s", OpcodeName(doPDU.Opcode()))
+				// Not our Data-Out — queue for later dispatch
+				s.pending = append(s.pending, doPDU)
+				continue
 			}
 			if err := collector.AddDataOut(doPDU); err != nil {
 				return err