fix(nbd): parse Request flags and type as separate u16 fields (#2548)

The NBD request wire layout (big-endian, 28 bytes) is `magic(4) |
flags(2) | type(2) | handle(8) | from(8) | length(4)`, but `dispatch.go`
was reading the four bytes after `magic` as a single `uint32` and
assigning it to `Request.Type`. That works only while the kernel chooses
`flags=0`; as soon as it sets any `NBD_CMD_FLAG_*` bit (e.g.
`NBD_CMD_FLAG_FUA` on a sync write) the high half leaks into `Type` and
the dispatch switch falls through to the default error path, killing the
NBD connection.

This PR splits `Flags` from `Type` into two `uint16` fields, decodes
each independently, and extracts the parsing into a small `parseRequest`
helper so the byte offsets are unit-testable. All command flags are
still intentionally ignored.

Production traffic on `main` is unaffected: the orchestrator does not
advertise `FlagSendTrim` or `FlagSendWriteZeroes`, so today's kernels
never set non-zero flags on the requests we receive. This is purely a
correctness fix to the wire parser so the dispatch keeps matching the
right command the moment any flag bit is set.

Author: ValentaTomas

packages/orchestrator/pkg/sandbox/nbd/dispatch.go

@@ -70,10 +70,18 @@ const (
 	NBDResponseMagic = 0x67446698
 )
 
-// NBD Request packet
+// NBD Request packet. Wire layout (big-endian, 28 bytes total):
+//
+//	magic(4) | flags(2) | type(2) | handle(8) | from(8) | length(4)
+//
+// Spec: https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md#request-message
+//
+// Flags carries the NBD_CMD_FLAG_* bits and is intentionally ignored — split
+// from Type so a non-zero flag bit doesn't corrupt the command opcode.
 type Request struct {
 	Magic  uint32
-	Type   uint32
+	Flags  uint16
+	Type   uint16
 	Handle uint64
 	From   uint64
 	Length uint32
@@ -189,7 +197,8 @@ func (d *Dispatch) Handle(ctx context.Context) error {
 
 			header := buffer[rp : rp+28]
 			request.Magic = binary.BigEndian.Uint32(header)
-			request.Type = binary.BigEndian.Uint32(header[4:8])
+			request.Flags = binary.BigEndian.Uint16(header[4:6])
+			request.Type = binary.BigEndian.Uint16(header[6:8])
 			request.Handle = binary.BigEndian.Uint64(header[8:16])
 			request.From = binary.BigEndian.Uint64(header[16:24])
 			request.Length = binary.BigEndian.Uint32(header[24:28])