cube(2): CUBE_OP_PUT takes a class mask

The mask is the writer's and is stamped once, at the moment the record's
class is known for certain; every later reader is spared re-deriving it. It
means nothing to this side — which bits are which class is a vocabulary's
business, and a kernel that interpreted one would be inventing a vocabulary.

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.
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.

The field is appended, so sizeof(cube_args) grows from 80 to 88 — still
distinct from the other three argument blocks, which is what the size-first
dispatch depends on.

include/uapi/linux/cube.h

@@ -34,6 +34,20 @@ 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.
+				 *
+				 * 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.
+				 */
+	__u16 reserved;		/* must be 0 */
 };
 
 #define CUBE_OP_PUT	1	/* store bytes at a coordinate */