netlink: use 48 byte ctx instead of 6 signed longs for callback

People are inclined to stuff random things into cb->args[n] because it
looks like an array of integers. Sometimes people even put u64s in there
with comments noting that a certain member takes up two slots. The
horror! Really this should mirror the usage of skb->cb, which are just
48 opaque bytes suitable for casting a struct. Then people can create
their usual casting macros for accessing strongly typed members of a
struct.

As a plus, this also gives us the same amount of space on 32bit and 64bit.
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netlink.h

@@ -192,7 +192,14 @@ struct netlink_callback {
 	bool			strict_check;
 	u16			answer_flags;
 	unsigned int		prev_seq, seq;
-	long			args[6];
+	union {
+		u8		ctx[48];
+
+		/* args is deprecated. Cast a struct over ctx instead
+		 * for proper type safety.
+		 */
+		long		args[6];
+	};
 };
 
 struct netlink_notify {