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.

drivers/cube/cube_syscall.c

@@ -32,7 +32,7 @@
 int cubelinux_kernel_put(const __u8 *space, __u64 x, __u64 y, __u64 z,
 			 const void *value, size_t len, __u16 flags);
 ssize_t cubelinux_kernel_get(const __u8 *space, __u64 x, __u64 y, __u64 z,
-			     void *buf, size_t len);
+			     void *buf, size_t len, __u16 *out_flags);
 int cubelinux_kernel_del(const __u8 *space, __u64 x, __u64 y, __u64 z);
 int cubelinux_kernel_sync(void);
 
@@ -196,7 +196,8 @@ static long cube_args_op(unsigned int op, void __user *uargs)
 		ssize_t got;
 
 		got = cubelinux_kernel_get(args.coord.space, args.coord.x,
-					   args.coord.y, args.coord.z, buf, args.len);
+					   args.coord.y, args.coord.z, buf, args.len,
+					   &args.flags);
 		if (got < 0) {
 			ret = got;
 			break;