cube(2): CUBE_OP_GET answers with the record's class

The last hole in the substrate: a caller could write a class through the syscall
but not read one back. `CUBE_OP_GET` now fills `args.flags` from the same index
entry the address came from, so learning what a record *is* costs nothing beyond
a read that was going to happen.

One field for both directions, because it is one thing — the class of this
record. A write states it, a read learns it, and neither is a special case of the
other. `find` hands the mask back with the address for the same reason: it is in
the same stride the binary search already read, so wanting both does not mean
searching twice. A read that finds nothing leaves 0 rather than a stale class for
the caller to believe.

The size of `cube_args` does not change, which matters because `size` is what
says which argument block arrived.

include/uapi/linux/cube.h

@@ -34,18 +34,21 @@ struct cube_args {
 				 * out: on -ERANGE, the bytes that would be needed;
 				 *      on success for CUBE_OP_GET, the bytes read.
 				 */
-	__u16 flags;		/* in: CUBE_OP_PUT only — the class mask to stamp on the
-				 * record. The mask is written where the record is written,
-				 * which is the moment its class is known for certain; a
-				 * reader never has to re-derive it. 0 is "no class", which
-				 * is what every record written before the field existed
-				 * reads as, so a caller that does not classify is not
-				 * writing a special value.
+	__u16 flags;		/* in: CUBE_OP_PUT — the class mask to stamp on the record;
+				 * out: CUBE_OP_GET — the mask the record carries.
 				 *
-				 * A store whose image is the legacy packed layout has no
-				 * field to put a mask in, and drops it: that layout cannot
-				 * carry a class and saying otherwise would be a lie about
-				 * the bytes on disk.
+				 * One field for both directions because it is one thing: the
+				 * class of this record. A write states it, a read learns it,
+				 * and neither is a special case of the other. 0 is "no class",
+				 * which is what every record written before the field existed
+				 * reads as — so not classifying is not writing a special value,
+				 * and a read that found nothing leaves 0 rather than a stale
+				 * class for the caller to believe.
+				 *
+				 * A store whose image is the legacy packed layout has no field
+				 * to put a mask in, and drops it: that layout cannot carry a
+				 * class, and saying otherwise would be a lie about the bytes on
+				 * disk. A read from such a store answers 0 for the same reason.
 				 */
 	__u16 reserved;		/* must be 0 */
 };