net: mdio: Fix lockdep falls positive splat

MDIO devices can be stacked upon each other. The current code supports
two levels, which until recently has been enough for a DSA mdio bus on
top of another bus. Now we have hardware which has an MDIO mux in the
middle.

Define an MDIO MUTEX class with three levels.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/mdio.h

@@ -13,6 +13,17 @@
 
 struct mii_bus;
 
+/* Multiple levels of nesting are possible. However typically this is
+ * limited to nested DSA like layer, a MUX layer, and the normal
+ * user. Instead of trying to handle the general case, just define
+ * these cases.
+ */
+enum mdio_mutex_lock_class {
+	MDIO_MUTEX_NORMAL,
+	MDIO_MUTEX_MUX,
+	MDIO_MUTEX_NESTED,
+};
+
 struct mdio_device {
 	struct device dev;